John McCain has been diagnosed with brain cancer.

The 80-year-old Rep. Senator for Arizona underwent surgery last week for what was thought to be a blood clot above his eye.

The Senate eagerly awaited his return and postponed voting on Trump's healthcare bill until he had come back to take part.

On Wednesday, McCain's family revealed that after the surgery, doctors discovered cancerous tissue in his brain.

'On Friday, July 14, Sen. John McCain underwent a procedure to remove a blood clot from above his left eye at Mayo Clinic Hospital in Phoenix.

'Subsequent tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot,' the family said in a statement.

President Trump, who last week joked about his 'crusty' condition when the senator's condition seemed less severe, sent his well wishes after learning of the development late on Wednesday night.

McCain's Fox News host daughter Meghan also shared an emotional message in which she described him as a 'warrior at dusk' who would not be defeated by the disease.

McCain, a Vietnam veteran who was held as a prisoner of war, has endured previous battles with skin cancer.

Scroll down for video

Arizona Senator John McCain (pictured on July 13, a day before undergoing surgery for a blood clot) has been diagnosed with brain cancer

There was no indication on the severity of his brain cancer on Wednesday but his office said it remained hopeful that treatment would be effective.

He told Rep. Rep. Lindsey Graham he planned to stay in hospital to undergo treatment after the news emerged, saying: 'I'm gonna stay here a little bit longer, take some treatments and I'll be back,' according to CNN.

Immediately, speculation rose over whether the disease may have impacted his confused questioning of ex-FBI director James Comey before the Senate Intelligence Committee in June.

In their statement, the family continued: 'He is in good spirits as he continues to recover at home with his family in Arizona.

'He is grateful to the doctors and staff at Mayo Clinic for their outstanding care, and is confident that any future treatment will be effective.

'Further consultations with Senator McCain’s Mayo Clinic care team will indicate when he will return to the United States Senate,' it said.

President Trump was among the first to offer his good wishes.

'Senator John McCain has always been a fighter. Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family. Get well soon,' he said.

It was a marked change in tone from his previous description of him as 'crusty'. Trump earlier said McCain, who spent five years as a prisoner of war in Vietnam, was not a war hero because he'd been captured.

President Trump was among the first to send his well wishes after learning the news on Wednesday

Barack Obama shared an emotional message for the 80-year-old after hearing the news himself

Clinton shared her own message for the senator who she described as 'as tough as they come'

Senate Majority Leader Mitch McConnell tweeted that McCain was 'a hero to our Conference and a hero to our country'

'My thoughts and prayers are with Senator John McCain and his family tonight,' New York Governor Andrew Cuomo tweeted

The 80-year-old's daughter, Fox News host Meghan, shared an emotional message after the news emerged on Wednesday night. The pair are pictured at the 2013 White House Correspondents' Dinner

Megan posted this photograph on Instagram after news of her father's condition emerged. She said her father was doing 'very, very well'

Barack Obama also shared a message for the senator after learning of his health battle on Wednesday night.

'John McCain is an American hero and one of the bravest fighters I've ever known.

'Cancer doesn't know what it's up against. Give it hell John,' he said.

McCain is pictured with his wife, Cindy, in Phoenix last year

The ordeal began when McCain, who has represented Arizona since 1987, went for his annual checkup, complaining of fatigue and double vision, CNN reported.

Doctors then performed a scan of the brain where they found a blood clot and removed it at the Mayo Clinic in Arizona last Friday.

Subsequent pathology tests confirmed the blood clot was associated with brain cancer.

Meaghan McCain, his Fox News host daughter, responded to the news in an emotional statement which she shared online.

In it, she described him as 'the toughest person I know'.

'The cruelest enemy could not break him. The aggressions of political life could not bend him.

'So he is meeting this challenge as he has every other. Cancer may afflict him in many ways: but it will not make him surrender. Nothing ever has,' she said.

The statement went on: 'He is a warrior at dusk, one of the greatest Americans of our age, and the worthy heir to his father’s and grandfather’s name. But to me he is something more. He is my strength, my example, my refuge, my confidante, my teacher, my rock, my hero — my dad.'

Last week, McCain's prognosis seemed less frightening.

Speculation has risen that Sen. McCain's health may have been behind his confused questioning of ousted FBI Director James Comey in June. At the time, he said he had been up late watching baseball the night before the hearing

Doctors said he'd endured a 'minimally invasive' procedure to remove the nearly two-inch clot and that the surgery went 'very well.'

POOR HEALTH MAY HAVE IMPACTED COMEY HEARING When McCain announced he was undergoing surgery to remove a blood clot last week, some people questioned whether his health played a role in his confused questioning of ex-FBI director James Comey in June. McCain dismissed initial concerns by ensuring he had just stayed up too late the night before watching baseball. However, his confusion could have been caused if a clot, called a subdural hematoma, was putting too much pressure on that part of his brain. That kind of clot could have been happened after a fall or if the Senator was taking a blood thinner, Dr Cisse said. He also explained that the left side of the brain there is specifically related to tasks such as speaking and relaying messages. 'This type of clot is often found in older people, because they do experience worse falls, and are often on blood thinners for something else,' Dr Cisse explained. 'They can develop slowly, over weeks or even months, and sometimes do not have any symptoms. But, if they grow to a certain size, the pressure on the brain can cause pain and even confusion or difficulty with everyday tasks such as speaking or moving around in patients.' Senator McCain has not previously disclosed whether he takes blood thinners. - Abigail Miller for DailyMail.com Advertisement

The surgery forced Majority Leader Mitch McConnell to delay a vote on the Republican health care bill, another setback for the effort to repeal and replace Barack Obama's Affordable Care Act.

It did, however, spark speculation that a health condition may have been behind his confused questioning of James Comey in June.

McCain appeared to confuse the ousted FBI director for the president and conflated the investigation into Russia - the subject of the hearing - with that of Hillary Clinton's use of a private email server when she was secretary of state.

'In the case of Hillary Clinton, you made the statement that there wasn't sufficient evidence to bring a suit against her, although it had been very careless in their behavior, but you did reach a conclusion, in that case that it was not necessary to further pursue her, yet at the same time in the case of Mr. Comey you said that there was not enough information to make a conclusion,' he said.

Later, he said: 'You're going to have to help me out here. In other words, we're complete, the investigation of anything that former secretary Clinton had to do with the campaign is over and we don't have to worry about it anymore?"

'I'm a little confused,' Comey replied.

Afterwards, McCain addressed the subject on Twitter after reading concerns about his health.

He said he'd stayed up late the night before the hearing to watch baseball and that his confusion was simply down to tiredness.

McCain, a former Navy pilot who was shot down over Vietnam and spent five-and-a-half years as a prisoner of war, was the GOP's presidential nominee in 2008.

He launched his career into politics in the 1980s after serving in the US Navy Airforce.

McCain spent five years as a prisoner of war after crashing his plane in Hanoi. He is pictured being treated by a Vietnamese doctor in 1967

Born into a military family, McCain prided himself on not having been the greatest student to ever walk the halls of the US Naval Academy in Annapolis, Maryland.

He graduated in 1958 and entered the Navy. By 1967, McCain was a veteran pilot aboard the USS Forrestal aircraft carrier off the coast of Vietnam.

He was preparing to take off on a bombing run over North Vietnam when disaster struck.

McCain is pictured being greeted by President Richard Nixon in 1973 after returning to the US

A missile accidentally fired from another plane, hitting the fuel tanks on McCain's aircraft and triggering explosions and fire.

McCain escaped from his plane by crawling onto the nose of the aircraft and diving on to the ship's fiery deck.

Before he could reach a fellow pilot whose flight suit was on fire, more explosions erupted, blowing McCain back.

When the inferno was finally contained 24 hours later, 134 men had been killed and hundreds more injured.

It was called the worst non-combat-related accident in US naval history.

Three months later McCain was on a bombing mission over Hanoi.

A missile struck his plane, shearing off the right wing. McCain ejected, and the force of the maneuver knocked him unconscious and broke both his arms and a leg.

He plunged into a Hanoi lake.

An angry mob dragged him from the water, broke his shoulder with a rifle butt and bayoneted him.

Fellow POWs at the Hanoi Hilton helped him survive, and he was imprisoned for 5-1/2 years. Videotape of his POW ordeal still exists.