Donald Trump's big problem isn't Jim Comey. It's that people don't trust him.

"James Comey vs. Donald Trump."

That's the obvious storyline ahead of the former FBI director's testimony in front of the Senate Intelligence Committee today -- especially after Comey's takedown of Trump in his prepared testimony released Wednesday.

And, to be sure, Trump has played up that "me vs. him" angle in the wake of his decision to fire Comey last month; he called the former FBI director a "showboat" and a "grandstander," not to mention suggesting that everyone in the FBI wanted Coney gone. (Acting FBI Director Andrew McCabe has said on several occasions under oath that he and the rank-and-file of the FBI respected and supported Comey.)

But Trump's real foe isn't Comey. It's an American public that is growing increasingly convinced that Trump simply isn't telling the whole truth about his motivations regarding the Russia investigation -- or much else.

In a new Washington Post-ABC News poll, just over one in three people (36%) said they trust what Trump says "a great deal" or a "good amount." Fifty-five percent trust Trump on Russia less than that or not at all. Almost half (49%) of all independents say they don't trust Trump at all when it comes to Russia. Significant majorities of the public also believe Trump is "interfering" with the Russia investigation (56%) and that he fired Comey to protect himself rather than for the good of the country (61%).

Trump's poll problems extend well beyond Russia. His job approval rating in a new Quinnipiac University national poll was 34% with 57% disapproving. More than twice as many people (51%) strongly disapprove of Trump as strongly approve of him (25%). Nearly six in ten (59%) say Trump is not honest -- including a majority (56%) of independents. Almost seven in 10 (68%) say Trump is not "level-headed."

You get the idea. Trump wasn't in great political shape when he won the White House. (The 2016 race was a rock-and-a-hard-place choice for lots and lots of people.) His first four months as president have been defined primarily by a series of self-inflicted wounds -- many having to do with the ongoing federal investigation, now being overseen by special counsel Robert Mueller, into Russia's meddling in the 2016 election and the possibility of collusion with the Trump campaign.

The unending cycle of negative headlines has taken a considerable toll on Trump. So much so that it's not clear that, beyond his political base, there are a whole lot of people who will believe whatever tweets, statements or messages Trump and his allies put forward today.

And, in truth, that credibility gap doesn't have all that much to do with Comey. It's almost entirely Trump's own fault. At every chance, he has worked to denigrate and dismiss the Russia investigation and those involved in investigating it. He's called it a "witch hunt," a "total hoax" and "fake news." While that may be music to his supporters' ears, it's increasingly hard for people outside of his base to listen to. (And, remember, his base isn't big enough to get him re-elected.)

Expect Trump to portray Comey's testimony as a personal vendetta by a guy he fired because he was bad at his job. But, if you scratch the surface of that Trump attack, you see that Comey isn't Trump's problem. Trump is Trump's problem.