The absence of credibility costs presidents dearly. And the bill for Donald Trump's years of falsehoods is now coming due.

Just four months into his presidency, Trump faces a mortal threat to his political survival. Contemporaneous memos kept by James Comey, the FBI director he fired last week, indicate Trump in February requested that he end an investigation into ex-national security advisor Michael Flynn.

If that happened, it could constitute obstruction of justice and fuel efforts to force his removal from office. The president, through a White House aide, said it didn't happen.

But few Americans believe the president's word anymore.

The man who touted "hyperbole" and "exaggeration" in his best-selling business book and seized the political spotlight with fabricated doubts about the President Barack Obama's birthplace, entered the presidency making transparently untrue statements about the size of his inauguration crowds. He holds the weakest reputation for honesty of any recent president.

Last month's NBC News/Wall Street Journal poll makes that clear. Only 25 percent of Americans rate Trump highly for being "honest and trustworthy."

Twice that many — 53 percent — give him low marks for honesty. That reflects condemnation of his truthfulness by virtually all Democrats and a solid majority of independents. Only a bare majority of Republicans — 53 percent — praise his truthfulness, even though more than 8 in 10 of them voted for him last fall.