There may be enough new pressure on Senate Republicans to allow witnesses at President Trump's impeachment trial, after the leak from a forthcoming book by former national security adviser John Bolton that contradicts what the White House has been telling the country.

Why it matters: This is a dramatic, 11th-hour inflection point for the trial, with an eyewitness rebuttal to Trump's claim that he never tied the hold-up of Ukrainian aid to investigations into Joe Biden.

GOP sources say the revelation could be enough to sway the four Republican senators needed for witnesses — especially since Sens. Mitt Romney of Utah and Susan Collins of Maine have already strongly signaled they’d vote for witnesses.

What happened: Bolton alleges in his book — "The Room Where It Happened," out March 17 — that Trump explicitly told him "he wanted to continue freezing $391 million in security assistance to Ukraine until officials there helped with investigations into Democrats including the Bidens," the N.Y. Times reported.

Trump strongly denied Bolton's claims on Twitter early today: "I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. ... If John Bolton said this, it was only to sell a book."

The state of play: Republican sources tell Axios that party leaders and the White House will still try to resist witnesses because, as one top aide put it, "there is a sense in the Senate that if one witness is allowed, the floodgates are open."

"If [Bolton] says stuff that implicates, say Mick [Mulvaney] or [Mike] Pompeo, then calls for them will intensify," the aide said.

What we can expect Trump's defense lawyers to say as they make their case at the trial, beginning at 1 p.m. today and continuing tomorrow:

They'll say Bolton's account doesn’t change any key facts, and reiterate that the aid, which was only briefly paused, was released without the announcement of any investigations.

They'll emphasize that Ukrainian President Volodymyr Zelensky said there was no pressure, the call record shows no linkage between the two, and Zelensky got his meeting with Trump at the UN.

They'll also argue that Trump’s concerns about corruption in Ukraine were well-known: He questioned giving aid to the country for a number of reasons, just as he has done with other countries.

The intrigue: Bolton submitted the book to the White House on Dec. 30 for a standard prepublication security review for classified information.

The Times notes: "The submission ... may have given Mr. Trump’s aides and lawyers direct insight into what Mr. Bolton would say if he were called to testify."

"It also intensified concerns among some of his advisers that they needed to block Mr. Bolton from testifying."

Between the lines: Trump's defense team has the advantage of being able to do triage at the trial for the next two days, while the House managers listen silently.

So Dems are making a public case, with Senate Minority Leader Chuck Schumer tweeting: "John Bolton has the evidence."

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