The best thing one could say about Donald Trump when it comes to the truth is that he has a bold disregard for facts.

He consistently lies, about matters both small (the crowd size at his inauguration) and large (millions of undocumented immigrants voted for Hillary Clinton). Americans know this, and they simply don’t trust Trump to tell the truth. But his well-documented record in this area didn’t prevent him from winning the election, and it hasn’t shaken his support from Republican members of Congress.

In other words, Trump has been able to find a political way forward even though he has established a reputation as someone who cannot be trusted to tell the truth. Lying, by itself, may never be enough to ruin him politically.

That’s how the question of Trump’s honesty has played out in the political context. The reality is that it is possible for elected officials to lie with impunity. Journalists are reluctant to call out even the most blatant of lies, and even if they do, voters may not be moved.

Legal world

The legal arena, however, is a different matter. The legal world is built on facts, evidence, proof and credibility determinations. Lying often has tangible and serious consequences.

Some of the scandals surrounding Trump’s presidency have already entered the legal realm — most notably, the Russia investigation and questions involving Trump’s decision to fire former FBI Director James Comey. Special counsel Robert Mueller is conducting an investigation, and congressional committees are holding hearings where witnesses testify under penalty of perjury.

In both of those settings — the Mueller probe and congressional hearings — facts and the truth matter. Mueller will make evidence-based decisions about whether to seek criminal charges against Trump associates including Paul Manafort, Michael Flynn, Jared Kushner and, possibly, Jeff Sessions or others. It is unlikely that Mueller could consider seeking criminal charges against Trump as a sitting president, but it is possible he could name Trump as an unindicted co-conspirator if the facts support this. Testimony before congressional committees could create stand-alone legal issues if witnesses do not testify truthfully by exposing them to possible perjury charges. Members of Congress could also conclude there is evidence to support impeachment charges.

Trump’s potential testimony

While Trump’s easy comfort with lying has served him well, or at least not sunk him, this trait in the political realm could be an enormous liability when it comes to the law. Trump has promised to testify under oath. His own history shows why this would be so dangerous: When Trump testified under oath several years ago in a civil case, he was forced to acknowledge some past lies, and also, according to Timothy L. O’Brien (the defendant in that case) “lied during the deposition” itself. (Trump lost the case, a libel lawsuit he had filed against O’Brien, who wrote a biography about Trump.)

If Trump follows through on his pledge to testify about Comey’s claims, he could be courting disaster. Comey has testified that Trump asked him for his loyalty and hoped he would find a way to let the Flynn investigation go. Trump has categorically denied both of those claims. Someone isn’t telling the truth here. If Trump repeats his denials under oath, the risk is that Mueller or members of Congress conclude he’s the one who’s lying, especially if written memos and other evidence support Comey’s account.

Even if Trump doesn’t testify, others have already done so and will continue to do so. Attorney General Sessions is scheduled to appear before a Senate committee on Tuesday. Just as with Trump himself, facts, evidence and evaluations of credibility will be centrally relevant.

Chris Edelson is an assistant professor of government in American University’s School of Public Affairs. His latest book, “Power Without Constraint: The Post 9/11 Presidency and National Security,” was published in May 2016 by the University of Wisconsin Press.