ANALYSIS: President Trump puts credibility on line in his biggest fight yet

— -- It may be that all the noise of the first 40-plus days of the Trump presidency – fights with the judicial and legislative branches, accusations hurled at national-security and diplomatic officials, “alternative facts” and the war on the “opposition party” of the press – built up to this moment.

Donald Trump is putting the credibility of his presidency on the line with an explosive allegation aimed squarely at his predecessor. He is asserting that President Obama ordered his phones at Trump Tower tapped during the election campaign, essentially accusing Obama of a felony and questioning the integrity of the FBI and the U.S. national-security apparatus.

The claim has every hallmark of a Trumpian move – bombastic, unsubstantiated, chilling in its implications and all wrapped up in a series of tweets. Trump’s assertion, posted on Twitter on Saturday morning, may even be an attempt to distract from the substance of the Russia controversy that’s swirling close to his inner orbit, with Attorney General Jeff Sessions forced step aside from involvement in any Justice Department probes into connections between Russia and the Trump campaign after Sessions was found to have been less than forthcoming with Congress.

This, however, was no casual tweetstorm. It has implications beyond any previous maneuver Trump has taken as a candidate or a president.

The story is rattling the very foundations of the U.S. government, with the potential for a constitutional crisis and major federal institutions diminished. It could easily eclipse even the Trump campaign’s alleged Russia ties as the defining scandal of the early months of the Trump presidency, leaving precious little space for lawmaking and coalition building.

With no evidence provided to back up his claim, Trump has already provoked a backlash from his own government. FBI Director James Comey has asked the Justice Department to state publicly that the president’s assertion is incorrect, ABC News has learned.

Comey’s request will produce either a stunning rebuke of Trump by his own Justice Department or a standoff that could end Comey’s tenure as FBI director, by either resignation or firing.

The careful and halting explanations for Trump’s words offered by White House officials hint at the discomfort ahead for virtually all involved.

Trump is “going off of information that he’s seen that has led him to believe that this is a very real potential,” White House spokeswoman Sarah Huckabee Sanders said of Trump’s claim about Obama on ABC’s “This Week” on Sunday. “And if it is, this is the greatest overreach and the greatest abuse of power that I think we have ever seen and a huge attack on democracy itself. And the American people have a right to know if this took place.”

Leaving aside the ironies of the White House relying on anonymously sourced news reports, which Trump condemns, and punting to Congress to investigate a claim made by the executive branch, Huckabee Sanders is correct: If what he said is true, it’s a massive scandal.

If it’s false, though, it’s just as big. If Trump thrives by undermining trust in institutions, he’s about to find out what happens when trust in his leadership is questioned in a sustained manner, even by people who ostensibly work for him.

It’s become a mantra that one mistake in trying to understand the phenomenon of Donald Trump is to take him literally but not seriously while his supporters take him seriously but not literally.

But if Trump wants to be taken seriously as president, he had better be literally correct in what he is alleging. His presidency can’t afford less than that now.