WASHINGTON (AP) — President Donald Trump's impeachment trial shifted swiftly to pointed, back-and-forth questioning Wednesday as Republicans strained to contain the fallout over John Bolton's forthcoming book, which threatens their hopes of ending the trial with a quick acquittal.

The day started simply enough. Three Republican senators asked Trump's legal team: If there was more than one motive for Trump's conduct in Ukraine, as he pushed for political investigations of Joe Biden, should the Senate still consider the Biden pressure an abuse of power?

White House lawyer Pat Philbin responded there's nothing wrong with the president acting on a personal as well as national interest. He declared the charge against Trump "absurd."

Democratic Leader Chuck Schumer sparked lively debate asking whether the Senate could really render a fair verdict without calling Bolton or acting White House Chief of Staff Mick Mulvaney to testify.

"There's no way to have a fair trial without witnesses," responded Rep. Adam Schiff, the Democrat leading the prosecution for the House.

"Don't wait for the book. Don't wait 'til March 17, when it is in black and white to find out the answer to your question," Schiff told the Senate.

That publication date is now in doubt. The White House on Wednesday released a letter to Bolton's attorney objecting to "significant amounts of classified information" in the manuscript, including at the top secret level. Bolton  and his attorney have insisted that the book does not contain any classified information.

The White House action could delay the book's publication if Bolton, who resigned last September — Trump says he was fired — is forced to revise his draft.

Wednesday's questions ping-ponged in a spirited hours-long debate, a last gasp at closing arguments from the House prosecutors and Trump's defense ahead of critical voting this week.

Fielding the written questions, Chief Justice John Roberts asked them of Trump's accusers and defenders.

Senate Majority Leader Mitch McConnell privately told senators he doesn't yet have the votes to brush back Democratic demands for witnesses now that revelations from Bolton have roiled the trial.

Republican ideas for dealing with Bolton and his book were fizzling almost as soon as they arose — among them, a witness "swap" with Democrats or issuing a subpoena for Bolton's manuscript.

GOP senators are sternly warned by party leaders that calling Bolton as a witness could entangle the trial in lengthy legal battles and delay Trump's expected acquittal.

Philbin made exactly that case in his response to Democrats' first question: "This institution will effectively be paralyzed for months on end," he said.

Republican Sen. Susan Collins of Main tried to give fresh momentum to a one-for-one witness deal saying it's "very important that there be fairness, that each side be able to select a witness or two." But Democrats dismissed those offers, especially as Republicans want to draw Joe Biden's son, Hunter, deeper into the proceedings.

"It's irrelevant. It's a distraction," said Schumer.

Bolton writes in a forthcoming book that Trump told him he wanted to withhold military aid from Ukraine until it helped with investigations into Democratic rival Joe Biden. That assertion, if true, would undercut a key defense argument and go to the heart of one of the two articles of impeachment against the president.

"I think Bolton probably has something to offer us," said Sen. Lisa Murkowski, R-Alaska. She met privately Wednesday with McConnell.

Trump disagreed in a tweet Wednesday in which he complained that Bolton, after he left the White House, "goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security."

The uncertainty about witnesses arises days before crucial votes on the issue. In a Senate split 53-47 in favor of Republicans, at least four GOP senators must join all Democrats to reach the 51 votes required to call witnesses, decide whom to call or do nearly anything else in the trial.

Collins, Murkowski and Utah Sen. Mitt Romney signaled an interest in calling Bolton or other witnesses and questions and answers at times appeared directed directly at them.

One Democrat, the centrist Sen. Joe Manchin of West Virginia, said he wouldn't have a problem hearing from Hunter Biden, who was on the board of a Ukrainian gas company, but doubted it will happen.

Most Republican senators don't want to call Bolton and most Democrats would rather avoid dragging the Bidens further into the impeachment proceedings. The Bidens were a focus of defense arguments though no evidence of wrongdoing has emerged.

One person watching from the sidelines Wednesday was Lev Parnas, the indicted associate of Trump lawyer Rudy Giuliani, who arrived at the Capitol saying, "I want to testify." Parnas, who has turned over evidence for the proceedings, cannot enter the Senate with his court-ordered electronic-tracking device.

Protesters swarmed the Capitol complex throughout the day, many demanding a fair trial.

The two days set aside for questions, Wednesday and Thursday, also allow each side more time to win over any undecided senators pondering the witness issue. In the meantime, all will have the opportunity to grill both the House Democrats prosecuting the case and the Republican president's defense team.

Trump faces charges from Democrats that he abused his power like no other president, jeopardizing Ukraine and U.S.-Ukraine relations by using the military aid as leverage while the vulnerable ally battled Russia. The second article of impeachment says Trump then obstructed the House probe in a way that threatened the nation's three-branch system of checks and balances.

Republican senators lobbed questions that furthered Trump's team legal argument that the House presented a shoddy case and that the president's actions are well within his rights and do not rise to impeachable offense.

Texas GOP Sen. Ted Cruz wanted to know, Does it matter if there was a quid pro quo?

Trump's celebrity attorney Alan Dershowitz argued that every president believes his interest and the public interest combine, and such quid pro quo's made in one's political interest are not necessarily corrupt.

"It cannot be impeachable if it's a mixed motive that combines personal interest and the public interest," Dershowitz told them.

Schiff's response mentioned one particular senator: He asked his audience to imagine what would have happened if then-President Barack Obama asked the Russians to dig up dirt on then-candidate Romney, the GOP's 2012 presidential nominee?

"All quid pro quos are fine?" Schiff asked. The next president, he said, "can ask for an investigation of you."

Far from trying to overturn the 2016 election as Trump's team argues, impeachment is needed to protect the 2020 election, Schiff argued.

The president's legal team tried to lock up its case Tuesday. Trump attorney Jay Sekulow addressed the Bolton controversy head-on in closing arguments by dismissing the former national security adviser's manuscript as "inadmissible."

Democrats say Trump's refusal to allow administration officials to testify only reinforces that the White House is hiding evidence. The White House has had Bolton's manuscript for about a month, but its release caught senators off guard.

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Associated Press writers Alan Fram, Mary Clare Jalonick, Andrew Taylor, Matthew Daly, Laurie Kellman and Padmananda Rama contributed to this report.