President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE and his longtime fixer Michael Cohen are waging a war over credibility — and the stakes could hardly be higher.

Cohen is reportedly willing to tell special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE that Trump was aware in advance of a pivotal June 2016 meeting attended by his son Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE, key campaign aides and a Russian lawyer. Trump Jr. had been promised dirt on Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE in the meeting.

The president is adamant that he did not know in advance about the encounter, a point he made again, in capital letters, in a Friday morning tweet.

The disagreement over the meeting could go to the heart of the central question Mueller is investigating — whether or not the Trump campaign colluded with Russia.

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But there will likely be plenty more discord between Cohen and Trump on other matters as well. Earlier this week, a tape of a conversation between the two men came to light. It seemed to have been surreptitiously made by Cohen.

Both men face significant challenges on the credibility front.

Trump’s propensity to exaggerate or give misleading statements is legion. A Washington Post analysis found that Trump made more than 2,000 false or misleading claims during his first year in office alone.

Cohen, for his part, once said he would “take a bullet” for Trump. He also has a history of bellicose interactions with reporters and others who challenged his then-boss.

Infamously, it was Cohen who arranged a payment to adult-film actress Stormy Daniels in the closing weeks of the 2016 campaign, apparently to stop her discussing an alleged sexual encounter with Trump back in 2006.

Those who have experience with both men are often skeptical about the veracity of either one.

Barbara Res, who served as an executive vice president of the Trump Organization, was asked by The Hill whether she considers Trump credible.

“How can you? Nobody does,” she replied.

Referring to his exaggerations and misstatements while in office, she added. “He used to do that with buildings. He would say people were buying apartments and we weren't even marketing them yet.”

Res noted that she had very few dealings with Cohen — except, she said, when she had written an article critical of the president during the 2016 campaign “and [Cohen] threatened me with a lawsuit.”

Longtime supporters of the president have been concerned from the moment Cohen’s home, office and hotel room were raided back in April.

The president himself reacted with particularly fierce criticism of prosecutors to those raids, while allies fretted behind the scenes about what Cohen might know — and whether investigators could persuade him to flip.

Now that he appears to be in the process of doing so, Trump loyalists are pouring scorn down on his head.

“He is a total scumbag who claims to be loyal but is a serial liar who will end up in jail,” was the verdict of one such loyalist, who spoke only on condition of anonymity.

Other prominent figures in the Trump-Cohen battle are also opening themselves up to credibility questions.

During an appearance on CNN on Thursday night, Trump lawyer and former New York City Mayor Rudy Giuliani called Cohen a "pathological liar” who had been “lying for years.” In May, Giuliani had called Cohen “an honest, honorable lawyer” during an ABC News interview.

Legal experts say that, from a courtroom perspective, issues of past credibility — or lack of it — matter less than whether there is evidence to support a particular charge.

“There is a big difference between credibility in day-to-day life and credibility as a witness in a legal proceeding,” said Joyce White Vance, a former U.S. attorney for the Northern District of Alabama.

In court, she added, someone’s credibility “is as good as the evidence that exists to corroborate their statements. So that’s the big question: can evidence be found to corroborate this claim [that Trump knew in advance about the June 2016 meeting]? Or is there evidence that corroborates Donald Trump’s claim that he didn’t know?”

Cohen has not been charged with any crime at all yet. His claims about what he knows are viewed by some within Trump’s orbit as bargaining chips intended to nudge Trump toward pardoning Cohen if any wrongdoing comes to light.

For now, there does not seem to be much chance of that happening. In the same Friday morning tweet from Trump in which he denied advance knowledge of the June 2016 meeting, the president implied that Cohen’s business interests in New York taxicabs were shady.

“Sounds to me like someone is trying to make up stories in order to get himself out of an unrelated jam (Taxi cabs maybe?),” the president wrote.

Vance noted that the battle currently being fought out could be more important from a public relations and political perspective than from a legal one.

Whether Trump or Cohen is more believable, she said, “matters very much in terms of public opinion."

“Rudy Giuliani seems to be fighting a battle to influence public opinion, knowing that down the road there might be impeachment proceedings. And perhaps [he is] wanting to convince the public that Michael Cohen cannot be believed so the public will call their congressperson saying, ‘Don’t vote to impeach the president.’ ”

Some people in the political world, however, are not sure how much public opinion can be changed, so deeply ingrained have views of Trump become.

Who is telling the truth matters “only if you are dealing in reality,” said former Republican National Committee communications director Doug Heye dryly.

“Quite often this entire scenario has been reality-free, especially if a Trump supporter has decided that they don’t care,” Heye added.

Whether that’s true or not, the battle for credibility will be waged fiercely in the days ahead.

The Memo is a reported column by Niall Stanage, primarily focused on Donald Trump’s presidency.