Arlington, Virginia (CNN) Not too long ago, voters in Maine and Alaska received a robocall from the man who owned the iconic moment of the 2016 Democratic National Convention.

"Hello, this is Khizr Khan," he said, "calling on behalf of the People for the American Way."

Khan was urging people to contact their senators -- Republicans Susan Collins and Lisa Murkowski -- and tell them to oppose one of President Donald Trump's judicial nominees, a man one preacher has called " moral poison ."

Thomas Farr, a North Carolina attorney, "has spent a career using the law to discriminate," Khan said on the call. "He's fought for laws to prevent African-Americans from voting, and made it harder for workers to stand up for their legal rights on the job."

If there were ever an example of what a political being Khan has become in the two years since he shot to fame, this was it.

He has been endorsing Democratic hopefuls, especially veterans, and appearing with them at campaign stops. He's made short videos that candidates have posted on their websites and aired on local television stations. Now he was familiar enough to make robocalls and to share headlines with powerful operatives.

"From Khizr Khan to Roger Stone, Idaho candidates get some big-name endorsements," read a headline in the Idaho Statesman.

Army Cpt. Humayun Khan's image looms large at the 2016 Democratic convention, where his father gave an impassioned speech.

Man, by nature, is a political animal, Aristotle surmised millennia ago. But Khan, despite his penchant for philosophy, would surely have laughed heartily at the notion of himself as one. He never expected to become an activist and such a well-known one at that, although he was wise enough to anticipate the trials and tribulations of fame.

A unique set of circumstances set Khan on his journey, some he could control and others that he could not. He calls it his destiny. Or as the Persian scholar and poet Rumi, whom Khan grew up reading, wrote: "Though destiny a hundred times waylays you, in the end it pitches a tent for you in heaven."

Khan has been waylaid a few times in his 68 years.

He arrived in America in late 1979, carrying a single silver Samsonite suitcase and $200 in his pocket. The life he led in his adopted homeland was, by all counts, quiet and simple, though it was far from plain. He was a hard-working Pakistani immigrant who had felt the sting of martial law and developed a deep love and respect for the Constitution of the United States; he eventually realized his dream of graduating from Harvard Law School. He, along with Ghazala, his wife of 43 years, raised three sons, one of whom deployed to Iraq as an Army captain and returned home in a coffin.

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Khan had never spoken publicly about his son's 2004 death except for a Washington Post story the following year. In it, he described Humayun as the comforter in the family who had taught disabled kids in high school how to swim. Khan took care not to express his opposition to George W. Bush's decision to invade Iraq.

But five days after a Muslim couple attacked and killed 14 people at a holiday party in San Bernardino, California, and 11 years, five months and 29 days after Humayun died, candidate Trump appeared on Khan's television screen. He was calling for a ban on Muslims entering the United States.

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Such a ban might have prevented Khan and his family from ever settling here. And what about Humayun's sacrifice? Was his service to the nation to be belittled in this way?

The next afternoon, Khan received a call from a reporter for Vocativ, an online storytelling site that offers its audience a "unique lens" into issues. The reporter told Khan that he knew about Humayun's death and wanted to get a response from Khan to Trump's comments.

No one had ever asked Khan to comment on a presidential candidate before. Khan was taken aback by the call, but nevertheless he agreed to the interview. He felt he could no longer remain quiet. Politics had suddenly taken a deeply personal turn.

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"We are proud American citizens," Khan said. "It's the values (of this country) that brought us here, not our religion. Trump's position on these issues do not represent those values."

Khan was asked about the San Bernardino shootings.

"This is the time for us American Muslims to rat out any traitor who walks amongst us," he said. "Among us hides the enemies of the value system of this country. And we need to defend it."

Hillary Clinton's campaign saw the Vocativ story and immediately called Khan to see if he was comfortable with Clinton mentioning Humayun in a speech she had planned on homeland security. Khan was OK with it if it were in the context of patriotism, not partisan politics.

Many months later, the Clinton campaign called again, this time to ask if Khan would speak at the convention.

Khan was not accustomed to airing his opinions on national matters and replied that he would have to think about it.

He understood the risk he would be taking by stepping onto the DNC stage. His friends and family had warned him that his privacy would be forever gone, that politics had turned ugly and he might even become the target of harassment or worse. He sensed his life might not ever be the same.

And for what? Would the words of a Muslim Gold Star father have any effect on an election that observers believed would be won handily by Clinton?

He wrestled with the request and deliberated in lawyerly fashion the arguments for and against his speaking publicly. He decided he would call the Clinton campaign and politely turn down the invitation.

But then he found a single envelope in his mailbox. There was no stamp, no return address. The handwriting belonged to a child.

"Dear Mr. Khan," the letter said. "You are a lawyer. Can you please not let them deport Maria? She is in fifth grade and she is our friend. Thank you."

Khan walked back to his house in Charlottesville, to a room set up to pay tribute to Humayun's life. He stared at his son's photograph, at his deep, dark eyes, and asked himself: "What would Humayun do?"

He and Ghazala both knew the answer. He called the Clinton campaign and said "yes."

Khan introduces Clinton at a New Hampshire rally in 2016. Her loss devastated him but strengthened his resolve to stay politically active.

He practiced his speech over and over again, and in the back seat of a taxi carrying him and Ghazala to the convention in Philadelphia he realized he had his pocket-size Constitution tucked in the left-breast pocket of his jacket, where it always was. He decided he would pull it out on stage.

The rest is now a part of political history. Khan delivered a seven-minute speech that rocked the convention and reverberated across America.

"Donald Trump, you are asking Americans to trust you with our their future. Let me ask you: Have you even read the United States Constitution? I will gladly lend you my copy," Khan said, waving the blue booklet in the air. "In this document, look for the words 'liberty' and 'equal protection of law.'

"Have you ever been to Arlington Cemetery? Go look at the graves of the brave patriots who died defending the United States of America -- you will see all faiths, genders and ethnicities. You have sacrificed nothing and no one."

The crowd roared. The moment went viral.

Other political neophytes might have relished their moment in the spotlight as the zenith of their lives and then moved on. In 2012, two families who belonged to Mitt Romney's church stood on stage at the Republican National Convention and talked about the candidate as a caring person. At the DNC, an Arizona woman spoke passionately about how Obamacare helped her daughter who was born with a congenital defect. They were ordinary people brought on stage to tell a heartwarming story to tout a candidate. They were not heard from again.

Khan meets with members of the Muslim community, and veterans, while campaigning for Clinton in Norfolk, Virginia.

Khan might have been one of them. But in those few minutes, his fiery speech made him a star and an avalanche of publicity followed. In the weeks leading up to the election, journalists sought him out for stories. He appeared on countless television shows, including numerous stints on CNN. He had become a voice to be reckoned with.

Needless to say, Trump's electoral victory devastated Khan. But it steeled his resolve to carry on.

Had Khan spoken out against a GOP candidate like George W. Bush, he might have considered returning home and disappearing from the public eye.

"But it continued to get worse," he told me recently. "What good is your patriotism if you don't speak when your country faces peril? When your Constitution is maligned? When the values you defend are maligned? When you have a choice to honor your sons and daughters and their sacrifice or you let their uniform be soiled by a corrupt bigot tainted by Vladimir Putin's corrupt money.

"He has been unfaithful to the Constitution of the United States," he said of Trump. "Therefore, I must speak."

In the months after the election, Khan penned a memoir in record time. " An American Family: A Memoir of Hope and Sacrifice" was published in the fall of 2017. It was a book that laid out Khan's love affair with America that began long before his celebrity moment at the convention.

"I am an American patriot," Khan wrote, "not because I was born here but because I was not. I embraced American freedoms, raised my children to cherish and revere them, lost a son who swore an oath to defend them, because I come from a place where they do not exist."

What good is your patriotism if you don't speak when your country faces peril? When your Constitution is maligned? When the values you defend are maligned? Khizr Khan

Critics praised the book. It could teach all Americans what real patriotism looks like, they said.

Khan embarked on a frenzied promotional tour that took him to 13 cities in a matter of weeks. He spoke at countless events while Trump continued to make news with controversial remarks and American politics just seemed to get uglier and uglier.

I wondered whether Khan had not been worn down by it all, whether he now regretted having made the decision to make that electrifying speech.

I went to his book signing event at a small church in Raleigh, North Carolina, after meeting him for the first time earlier that November afternoon. For all the power of his words, Khan is a rather demure man and soft-spoken. He rarely raises his voice, though his language about Trump has certainly gotten fiercer over time.

I watched people listen to him intently and then tell him afterward they were sorry for his loss and that he had touched their lives. They came carrying copies of the memoir as well as Khan's first book on the Constitution, written for children.

By the time I saw him again in Atlanta a few weeks later, his voice had grown hoarse and he was visibly tired. In all, he counted 176 zigzag speaking engagements across the country in 2017. On a December night in San Francisco, he fell ill with pneumonia in his hotel room. But he still had appearances left in Arizona and New York. He popped two kinds of antibiotics and fell on his bed every night. He wanted to keep going. He was a humble man who told me he could not bear to disappoint his audiences.

This year has hardly been any less stressful.

He spoke at an event honoring Japanese-Americans who were forced into internment camps during World War II. At a diversity festival in Charlottesville, he stood up with Susan Bro, the mother of Heather Heyer , who was killed after a car plowed into a crowd of counterprotesters at a white nationalist rally. He jumped into the fray after Trump offended the family of a soldier killed in Niger. He began campaigning for Democrats who are challenging Republicans in the midterm elections. The list of candidates he's supporting has grown so long that he can hardly recite all the names anymore.

And in March, he wrote a personal appeal to the Supreme Court to strike down the very policy that propelled him on this life trajectory: Trump's travel ban. He felt compelled to do this by his unwavering, lifelong devotion to the Constitution.

A room at the Khans' house in Charlottesville is dedicated to the son they lost. For years, they kept their intense grief private.

On the morning of June 8, 2004, Capt. Humayun Khan was supervising a checkpoint outside Camp Warhorse near the central Iraqi city of Baquba. A taxi approached the gates, but Humayun wanted to make sure the driver was not confused and did not order his soldiers to put a .50-caliber shell through the windshield. Instead he ordered his soldiers to hit the dirt, and he began to move toward the taxi to try and stop it. He was killed when suicide bombers in the taxi detonated their explosives.

Khan and his wife moved from Maryland to Charlottesville after Humayun's death so they could be close to their other two sons. They started a memorial award in their son's name for the University of Virginia ROTC program, where Humayun began his military career.

Each year at the commissioning ceremonies, Khan hands America's newest officers pocket-size copies of the Constitution and reminds them to think hard about their oath to defend it, he told the justices in his brief.

"My son died for that document," he wrote.

Khan argued that Trump's travel ban desecrated his son's service and sacrifice as a Muslim-American officer. He argued that it also violated his own constitutional rights.

"The message," he said, "is that Muslims are unwelcome outsiders."

An honor no one wants

On a Saturday morning in the spring, after Washington's Yoshino cherry blossoms have already peaked and four days before the Supreme Court hearing on Trump's travel ban, Khan arrived at my hotel to pick me up in his 2018 Honda Accord. The odometer had already clocked 25,000 miles, mostly earned from his drives back and forth from his rented apartment in Alexandria to Charlottesville.

I was seeing Khan for the first time without his usual suit and tie; he wore a black turtleneck and a tweed jacket. But on his lapel was the pin he always wears close to his heart. It's a pin of honor that no one wants: the Gold Star.

Khan adjusted his round spectacles and cleared his throat before beginning our short drive, past the Iwo Jima Memorial and alongside the Potomac to Arlington National Cemetery. Khan can see the river from his apartment balcony, and he often walks along it. Gazing out at the water gives him solace.

He needs those calming moments. His life was upended when Humayun was killed in 2004. It was upended again when he ascended to national prominence.

The attacks on Khan started almost immediately with Trump himself, who belittled Ghazala for not speaking on stage, suggesting she was a subservient Muslim woman. Trump even said that the Clinton campaign had penned the speech Khan so painstakingly wrote and practiced repeatedly.

Khan addresses activists and reporters at the Supreme Court in April, when the justices heard arguments on the travel ban.

Trump adviser Roger Stone added to the fire by saying Khan was part of the Muslim Brotherhood. The far-right Breitbart News said Khan believed the US Constitution was subordinate to Sharia or Islamic law.

Khan is acutely aware of all the people out there who might want to hurt him. He has taken precautions such as putting up "no trespassing" signs in his front yard and other measures he did not want to divulge. He said he lives not in fear but with concern.

"I keep my eyes open," he said.

"Those 56 people who signed the Declaration of Independence -- they were the real heroes," he continued. "They put their name, their state, their city so that the British could invade their properties, their homes, raid their households. Some were arrested, some were charged with treason. Their properties were confiscated. They did not say, 'Let's leave that anonymous.' They put their names down."

He paused for a moment and said, "But I am careful. That's about it."

Khan's life got so much more complicated with his activism. I asked him where he draws his strength from, and what he does to relax.

"Well, I like to read," he said.

I assumed he would name Rumi, which he did, or another familiar author, but instead I learned he was engrossed in a history of the Bill of Rights, "Bills, Quills and Stills."

Sometimes, he watches television shows other than news. "You know that one about the nerdy science kids," he said, racking his brain to remember the name of "The Big Bang Theory."

And sometimes, he drops in at La Madeleine in Old Town Alexandria for a cranberry and pecan salad. He likes their croissants. "They are baked fresh right here," he said.

But his activities were interrupted ahead of the Supreme Court's oral arguments on the Trump travel ban. Having written his amicus brief, he wanted to attend -- in case the justices asked him any questions. He spent countless hours meeting with his lawyer.

"The outside world doesn't know how much preparation it takes to appear before the Supreme Court," he said. "Judges ask all sorts of questions. I have to be fully prepared."

He told me he was nervous; the hearing meant everything to Khan.

"Trump is fulfilling his campaign promise. That is why this Muslim ban is being issued," he said. "He has no moral courage."

Khan has no desire to criticize a sitting president, he said, unless that president is threatening American democracy.

Khan cited Trump's attacks on the media. The first thing authoritarian rulers do is shut down the free press, he said. The second thing is they don't like the rule of law, he said, referring to the administration's prickly relationship with the FBI and CIA.

"I have lived under martial law twice," he said of his younger years in Pakistan. "I know how it feels when authorities say, 'I don't like the newspapers, turn them off. I don't like the radio or television stations. Turn them off. Only my voice will prevail.'

"I know how it feels to lose these freedoms and liberties," he said.

Khan campaigns for Democrat Phil Murphy in the New Jersey governor's race in October. Murphy won with 56% of the vote.

We approached the main gate to Arlington National Cemetery. The drill for visiting families became familiar to Khan many years ago. He visits Humayun's grave on special occasions such as the anniversaries of his birth and death. He visits Humayun's grave on ordinary days when he just wants to be near his son.

Khan reached over, opened the glove box and pulled out a permanent parking pass that he displayed on the windshield. The guards waved him through.

I peered through the windows at the tall trees bursting with buds, signs of new life hanging over the dead. The shadows of the barer branches looked like black lace draping rows and rows of white gravestones.

I know several soldiers and Marines who are buried at Arlington. I know wives and husbands, daughters and sons and mothers and fathers, like Khan, whose loved ones found their final resting place here. I glanced at Khan in the driver's seat and suddenly the weight of the cemetery bore down on me.

Khan steered the car toward Section 60, where many of the dead from America's newest wars -- Afghanistan and Iraq -- are buried. Not too long ago, he was at Union Station buying a train ticket when the man behind the counter recognized him. He came out and gave Khan a hug. He, too, had lost a son.

The man told Khan: It creates a hole in your heart that is never filled. So be strong. Then he issued Khan the ticket.

We walked toward Grave No. 7986 -- his son's grave. We passed headstones of soldiers and Marines of all religions and ethnicities, a representation of American diversity.

Barun Rai, Michael Yury Tarlavsky, Francis Chinomso Obaji, Alan Dinh Lam, Tulsa Tulaga Tuliau, Nicholas S. O'Brien, Michael Luis Gonzalez, Nicholas Lee Ziolkowski, Brian Anthony Medina, Ayman Abdelrahaman Taha.

Latinos, Arabs, Asians, African-Americans. Christians, Jews, Hindus, Buddhists and Muslims.

Ziolkowski, a corporal, was 22 when he died. Someone had left birthday balloons for him on the day we visited.

Medina, a Marine lance corporal, was only 19 when he was killed in Falluja several months after Humayun's death. Khan reached out to Medina's father.

Khan often stops to look at all the things people have left at graves. Laminated letters, medals, photographs, keepsakes and religious objects. He utters the names of the dead out loud.

"How precious is our freedom," he said, picking up a 5-by-7 color photo of Robert J. Hess next to his headstone. "That's why I pay very close attention to the date of birth and date of death. How young they were. How full of life."

A group of young Navy cadets broke our solitude that day at Arlington. They were among the thousands of people who had come to tour America's most hallowed ground.

Khan contends Trump has little sympathy for Gold Star families, little respect for men and women in uniform. He has made that point repeatedly in his speeches and television appearances. That morning, as we walked the aisles of Section 60, Khan found it hard to remain restrained.

"Drag that SOB out of the White House and bring him here."

Khan often visits his son's grave at Arlington National Cemetery. When faced with tough decisions, Khan asks: What would Humayun have done?

With bowed heads, we finally stood before Humayun Saqib Muazzam Khan. Cpt. US Army. Born on September 9, 1976. Killed on June 8, 2004. Interred on June 15, 2004.

Khan straightened a small container of canary yellow mums and placed a small stone at the top of the headstone, a sign of visitation and respect.

He visits Humayun these days not just as his father but also as an activist committed to doing right by his son. He has no regrets about the direction his life took after his speech at the convention and has fully accepted his role as the activist he has become. But even now, he feels unprepared for the limelight, unqualified to take on his destiny.

But here, on this one small patch of American soil, few would dare to challenge him.

Epilogue

On April 25, Khan donned a navy blue suit, pinned his Gold Star on the lapel and made his way to the Supreme Court for the 10 a.m. hearing on Case No. 17-965, Trump, President of US v. Hawaii. The justices did not ask him any questions, though he arrived fully prepared.

Afterward, he stood on the steps of the highest court of the land and addressed the crowd.

"Our case was very vigorously presented and we await a positive outcome," he said. "But what was affirmed to all of us is that no one, even the president of the United States, is above the law."

Then he returned to his work in support of Democratic candidates across the nation and remained hopeful that the justices would revoke Trump's travel ban.

A little over two weeks after the anniversary of Humayun's June 8 death, Khan was campaigning in Minneapolis when the Supreme Court issued its ruling. By a 5-4 vote, the justices upheld the travel ban . It was a huge victory for Trump, another crushing blow for Khan.

But he found some comfort in the passionate dissent by Justice Sonia Sotomayor. She compared the court's opinion to one issued in 1944 that endorsed the internment of Japanese-Americans during World War II. Khan told me he was reminded also of the infamous pro-slavery ruling in the 1857 Dred Scott case. Both those decisions are now regarded as low points for the highest court of the land.

They are looked upon now "as a national disgrace and violations of the nation's democratic values under the disguise of the national security," Khan said. "So will this decision."

Trump won, Khan said, because the Republicans in Congress successfully torpedoed Barack Obama's nomination of Merrick Garland for the court. Instead, the newest justice, Neil Gorsuch, added a conservative voice on the bench.

"What else I can say?" Khan said. "Trump's judge sitting on Judge Garland's stolen chair gave the majority courage and words to trample upon our constitutional values."

Again, Khan's resolve steeled.

He was off to Michigan when we last exchanged emails in early July. Republicans, he told me, had abdicated their adherence to the rule of law and this nation's system of governance to line up behind Trump. This made Khan more passionate than ever about dismantling the GOP majority in Congress. It was, he felt, the only way left to preserve American democracy.