Mr. Trump’s presidency has been marked from the start with false or misleading statements, such as his outlandish claims that more people came to his inauguration than any before and that at least three million unauthorized immigrants voted illegally against him, costing him the popular vote. He has gone on to assert that President Barack Obama wiretapped Trump Tower, a claim that his own Justice Department refuted, and that he would not benefit from his tax-cutting plan.

The lack of fidelity to facts has real-world consequences in both foreign affairs and domestic policymaking. Foreign diplomats and lawmakers of both parties say they do not assume anything he says is necessarily true. In a White House where one aide described the existence of “alternative facts” and another acknowledged telling “white lies,” staff members scramble to defend his claims without putting their own credibility on the line. News organizations debate when to use the word “lie” because it implies intent.

Since Mr. Trump became a presidential candidate, PolitiFact has evaluated more than 500 assertions and found 69 percent of them mostly false, false or “pants on fire” false.