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On a hot midsummer morning in Indianapolis, Joe Biden took the stage at a Democratic presidential candidates’ forum and turned his attention toward Russian election interference.

Biden assailed President Trump for having said he would accept help from a foreign government. “It is outrageous, it is un-American and it’s close to treasonous,” the former vice president thundered.

Little did he know that Trump is alleged to have done just that — asking Ukraine to help him dig up dirt on Biden himself.

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About an hour before Biden addressed the National Urban League, Trump had ended his now-infamous July 25 phone call with Ukrainian President Volodymyr Zelensky in which he asked Zelensky to open an investigation into alleged corruption by Biden and his son Hunter.

“It sounds horrible to me,” Trump said, referring to the Bidens.

The events that morning — separated by an hour on the clock and nearly 600 miles on the map — rose out of a months-long fixation that each man had with the other, putting their respective orbits on a collision course and imperiling their 2020 ambitions.

The extraordinary five months since then have brought into fresh relief the extent to which Trump would go to target Biden, seemingly unable to resist endangering his presidency by pursuing the falsehoods and conspiracy theories that appear to have consumed him.

“Some days it literally does shock me that the only reason Donald Trump is embroiled in this impeachment inquiry is because of the actions he took trying to take down an opponent in the race,” said Symone Sanders, a senior Biden campaign adviser. “That’s what the impetus was. Donald Trump only did it because of Joe Biden.”

© Jabin Botsford/The Washington Post President Trump addresses a small-business roundtable at the White House on Dec. 6.

The clash marks the first major engagement between the two possible presidential rivals, providing a potential preview of a 2020 contest that is likely to be conducted without the traditional guardrails that limited past campaigns.

From the point of view of the Biden camp, the former vice president has shown his ability to stand toe to toe with Trump — eventually finding his footing and showing he could withstand sustained Republican attacks. With about a month to go before the first votes in the Democratic primary, Biden has remained at the head of the field in most public polls.

But Trump’s defenders believe Biden has been badly damaged by the questions they have raised about his involvement in Ukraine, arguing he has been redefined as a compromised politician rather than the partner of a popular former president.

The dynamic has also thrust Biden’s relationship with his troubled son into public view — highlighting a painful, messy chapter for which he has provided no easy answers. In a Democratic presidential debate on Dec. 19, Biden delivered a defiant line that his campaign advisers thought was the best of his six debates: “If anyone has reason to be angry with the Republicans and not want to cooperate, it’s me — the way they’ve attacked me, my son, my family. I have no love.”

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Biden launched his candidacy by decrying Trump’s immorality, inviting the president’s wrath as a strategy to stand out in the crowded primary field and cast himself as the all-but-presumptive nominee. But he and his advisers did not foresee the degree to which Trump would demonize Biden and his family, nor that it would boomerang back on the campaign with unpredictable consequences, testing its ability to admit mistakes and combat the misinformation and innuendo spewing from the presidential megaphone.

“Trump’s attacking Biden is a double-edged sword,” said Scott Reed, a veteran Republican strategist who works for the U.S. Chamber of Commerce. “On the one side, it hurts and raises Biden’s negatives. On the other side, it puts him on the same level as the president of the United States, and that would help him inside his primary if he were saying something strategic to capitalize on it.”

Trump advisers saw the Hunter story as Biden’s soft underbelly — an easy vulnerability for Trump to exploit in a way that they hope redefines him. “Joe Biden is now viewed more as Hunter’s father than Obama’s vice president,” said Kellyanne Conway, counselor to the president.

For Trump, his fixation on Biden set him on a path to becoming the third U.S. president in history to be impeached and to face trial in the Senate. Trump’s preoccupation also showcases his appetite for unfounded conspiracies, as well as his eagerness to crash through the traditional guardrails that otherwise might have contained his more dangerous or self-sabotaging instincts.

In Biden, Trump and his allies saw a political threat — a Democrat with blue-collar bona fides who could cut into the president’s winning coalition of working-class whites.

Trump’s North Star has been undoing the legacy of former president Barack Obama, and he and his allies set out to help achieve that than to emasculate his predecessor’s vice president, whose campaign has promised an Obama restoration.

An early round of Trump campaign internal polling — which ultimately leaked out into news reports — showed Biden trouncing Trump in key swing states, further heightening the Trump campaign’s early concern that Biden posed the biggest danger to the president’s reelection chances.

John McLaughlin, one of the Trump campaign’s pollsters, recalled telling the president around this time that Biden would not remain as potent a threat as he appeared this past spring.

“The ironic part about the Biden candidacy is I always thought he was a lot weaker and more vulnerable than people thought and that his best day would be his first day,” McLaughlin said.

© Jabin Botsford/The Washington Post Trump talks to journalists on the South Lawn of the White House on Dec. 2.

Many in Trump’s orbit say that although they most feared Biden in theory, as soon as “Sleepy Joe” — as the president dubbed him — entered the race, he quickly lost some of his luster amid stumbles and a sense that he wasn’t the nimble politician he had once been.

Yet Trump’s focus on Biden persisted, they said, borne out of the president’s obsession with the 2016 election and its aftermath. While he had been the subject of what he called myriad “witch hunt” investigations — from special counsel Robert S. Mueller III’s report into Russian interference to probes of his family’s businesses — he felt others, such as Biden, were getting a pass.

Hunter Biden had served on the board of Burisma, a Ukrainian energy company, and pocketed a large salary while his father was vice president. Although there is no evidence of wrongdoing on the part of either Biden, Trump viewed the situation as a double standard.

Trump registered a similar complaint on the sidelines of the U.N. General Assembly in September, telling reporters during a meeting with Polish President Andrzej Duda, “If a Republican ever did what Joe Biden did, if a Republican ever said what Joe Biden said, they’d be getting the electric chair by right now.”

“Look at the double standards,” Trump concluded. “You people ought to be ashamed of yourself.”

‘Yeah, that was a warning’

At the start of the year, as Biden was preparing to enter the presidential race and weighing whether his family could withstand the scrutiny, Trump’s personal attorney Rudolph W. Giuliani was busy with a different set of concerns.

Giuliani spent those weeks meeting with Ukrainian officials, forming the foundation for the unfounded claims against the Bidens he would propagate later in the year. He spoke with Viktor Shokin, the top prosecutor who was ousted in early 2016 after Biden pressured then-Ukrainian President Petro Poroshenko by linking U.S. aid to Shokin’s removal.

Biden, then the vice president and acting on behalf of Western allies, had come to believe that Shokin was not doing enough to crack down on corruption in the country. But Giuliani smelled a conspiracy. Although his foreign adventures began as an effort to exonerate Trump from allegations of Russian collusion, Giuliani said he believed he had uncovered something far more nefarious on the part of the Bidens.

“When I got this evidence of Ukrainian collusion, where they mentioned that Joe Biden was involved in developing some of the collusion, I jumped on it, and I started to find people in Ukraine who were willing to come over and talk to me about it,” Giuliani said in a November interview with conservative host Glenn Beck of Blaze TV.

“It turns out the Trump story’s not true — and the Biden story is true,” Giuliani added. “And they didn’t cover it! Not even a whimper!”

© Jabin Botsford/The Washington Post Rudolph W. Giuliani greets supporters before a Trump campaign rally in Manchester, N.H., on Aug. 15.

Some of the early public signs of where Giuliani was heading came on April 1, when conservative columnist John Solomon wrote a piece in the Hill — headlined “Joe Biden’s 2020 Ukrainian nightmare: A closed probe is revived” — that focused on Hunter Biden and Burisma. It included interviews with Shokin and then-Ukrainian Prosecutor Yuri Lutsenko, indicating that some Ukrainians with axes to grind were eager to publicly fan their grievances. A few days later, Solomon penned another contribution that mentioned Giuliani’s interest in the topic — a tipoff that Trump’s team was involved.

These early warning signs did not alarm Biden’s senior advisers, who largely dismissed Solomon’s stories as fodder for the right-wing echo chamber. Many advisers in both the Biden and Trump camps requested anonymity in order to share candid assessments of the rivalry.

“It’s easy to say, ‘Yeah, that was a warning,’ ” said one Biden adviser. "But I didn’t think that’s where they were collectively going. I thought it was a small subset of right-wing folks testing things out. But I did not think that was their game plan.”

Trump, for his part, proved an eager audience when Giuliani returned from Ukraine, bearing conspiratorial tales like so many souvenirs from abroad. “I am 1,000 percent sure Rudy got Trump spun up,” said a former White House official.

‘Going at Trump’

As Biden began planning his presidential campaign, his advisers decided on an approach that was different from most of his rivals. They wanted to goad Trump from the start — to launch a campaign that was almost designed to troll him, and to start a primary campaign that instead felt like the general election.

For Biden, who in 2016 decided not to run and envisioned political retirement, it was Trump who became the ostensible reason Biden got into the 2020 race.

“Going at Trump from the outset was a way to ensure this race was in large part about Biden versus Trump,” a senior Biden adviser said. “Anytime this race is on the battlefield of Trump versus Biden, that’s really good for our campaign.”

Biden’s aides were initially taken aback — and delighted — by how much Trump played into their strategy. After one of Biden’s first speeches, in a union hall in Pittsburgh where he got the endorsement of the International Association of Fire Fighters, Trump began retweeting messages from random firefighters who did not support Biden.

© Daniel Carde/AFP/Getty Images Democratic presidential candidate Joe Biden campaigns in San Antonio on Dec. 13.

Biden was almost giddy. “I understand the president’s been tweeting a lot about me this morning. I wonder why the hell he’s doing that!” he said in Iowa City. “Yo yo, woah woah woah. Anyway. I’m going to be the object of his attention for a while, folks.”

Inside Biden’s campaign headquarters in Philadelphia, where there is a cardboard cutout of the former vice president but no imagery of Trump, his campaign each day would try to find ways to keep the head-to-head fight simmering. When Biden and Trump both went to Iowa on the same day, Biden’s campaign released his full prepared remarks — all 2,887 words — at 6 a.m. to attempt to set the cable news agenda.

Biden tried to come up with his own play on Trump’s “Make America Great Again” slogan, saying, “Let America be America Again.”

He mocked Trump’s nickname for him: “I think I’m either low IQ or slow. Slow Joe Biden?”

Trump, meanwhile, called Biden a “dummy” and “the weakest mentally.”

“He looks different than he used to,” the president opined. “He acts different than he used to. He’s even slower than he used to be.”

Sanders, of the Biden campaign, said: “We knew he was obsessed when he wouldn’t stop tweeting. It was apparent very early that the VP occupied a lot of real estate in Donald Trump’s head.”

After a summer that largely turned inward, as Biden fought less with Trump and more with his fellow Democrats, the return of Trump’s focus came with a bang: news of a whistleblower’s report alleging that Trump had asked Ukraine to investigate the Bidens and had tied up foreign aid.

© Toni L. Sandys/The Washington Post Biden poses for photos onstage at the Democratic debate on Nov. 20 in Atlanta.

Even for a campaign that was built to focus on Trump, and a candidate who had said he was prepared for any dirty tactics the president might bring, they were in disbelief. In part because they wanted firm confirmation of the allegations, the campaign was slow to respond — “It was too crazy to be real,” in the words of one Biden adviser — and Biden was reluctant to comment initially, simply saying that he did nothing wrong and neither did Hunter.

After a long day of texts, emails and calls among Biden campaign advisers assessing the rapidly shifting news environment, Sanders sat in her hotel room and watched a Republican on cable news raise questions about whether Biden had intervened in Ukraine to help Hunter, as the news anchor did not challenge the notion.

Realizing that the campaign was at risk of losing the battle of public perception, top advisers started pushing back more aggressively, sending out mass emails challenging reports — and reporters — they viewed as unfair.

Biden was angry. On a call with senior staff the next morning, he said that he wanted to make the point that Trump was afraid of him, but also that he was furious about the abuse of power. Biden chided a Fox reporter for asking about him about Hunter, saying “Ask the right question!” He also said that Trump was scared because he would “beat him like a drum.”

It was a ’50s-style bit of Biden trash talk. But his delivery was more forceful than before, and it provided enough of a rallying cry for his campaign that some of his aides began changing their Twitter profiles to include an image of a drum with two drumsticks.

‘Stumbles and bumbles’

Trump allies say Biden didn’t occupy space so much in the president’s head as he did on his Twitter feed and television screen, and they argued that many of his attacks were ones of opportunity rising out of media reports on Biden and his son.

In June, ABC News aired a report from Tom Llamas, who had traveled to Ukraine and asked, “Was Hunter Biden profiting off his dad’s work as vice president, and did Joe Biden allow it?” One Republican close to the Trump campaign said the television segment served as the “definitive” dossier in doing opposition research on Hunter.

Then, in July, a thick New Yorker issue featured a lengthy profile of Hunter in which the tagline referenced “scrutiny for his business dealings” and a “tumultuous personal life.” The headline posed the thorny question also facing Biden aides: “Will Hunter Biden jeopardize his father’s campaign?”

© Stephanie Keith/Reuters Trump and Vice President Pence address a campaign rally in Hershey, Pa., on Dec. 10.

Conway carried the issue into the Oval Office with her and showed it to Trump. After initially believing Biden could be a potent challenger, Conway said his “initial stumbles and bumbles and lost opportunities” undermined what he had tried to portray as his “inevitable electable candidacy.”

While Biden aides point to his resiliency under repeated attacks from Trump and his fellow Democrats, some Trump allies worry that the president and his team have not adjusted to the reality that Biden is not a sure bet for the nomination. Some inside and outside the administration say Trump’s team should be turning its focus to other Democrats surging now, or those who could have a breakout moment and prove tough to beat in the general election.

Tim Murtaugh, the Trump campaign’s communications director, responded to this critique in an email: “We have never been convinced that Joe Biden would be the eventual Democrat nominee. It’s just that he presents such irresistible opportunities for criticism and mockery.”

So it was earlier this month at a raucous rally in Hershey, Pa., where Trump kept his focus trained on Biden despite polls showing fluid standings in the top tier of Democratic candidates.

“Had you ever noticed where Biden keeps saying he’s in the wrong state?” Trump asked his supporters. “Like, if he’s in Ohio, ‘It’s great to be in Iowa tonight!’ If he’s in Pennsylvania, ‘It’s wonderful to be in the state of Delaware!’ What is wrong with this guy? What’s wrong with him? There’s something wrong, okay? There’s something wrong.”

philip.rucker@washpost.com

ashley.parker@washpost.com