WATCH: Senators ask questions during President Donald Trump’s impeachment trial

WASHINGTON (AP) - President Donald Trump's impeachment trial is shifting to questions from senators, a pivotal juncture as Republicans lack the votes to block witnesses and face a potential setback in their hope of ending the trial with a quick acquittal.

WATCH:

Despite Trump's defense team's plea for it to “end now,” 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 John Bolton, the former national security adviser, have roiled the trial.
 
Republican senators are trying to figure out a way to to deal with fallout from Bolton's forthcoming book, which provides a potential eyewitness account of Trump's actions at the heart of the impeachment charges.
 
But ideas being floated are fizzling almost as soon as they arise - 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.
 
One key Republican, Sen. Susan Collins of Maine, gave fresh momentum Wednesday 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 Senate Democratic leader Chuck 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.
 
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 White House on Wednesday sent a letter  to Bolton's attorney objecting to “significant amounts of classified information" in the manuscript, including at the Top Secret level. The former national security adviser and his attorney have insisted that the book does not contain any classified information.
 
The finding could delay the book's publication if Bolton is forced to revise the draft.
 
Bolton left the White House last September and says he resigned. Trump says he was fired.
 
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. Several Republicans apparently are ready to join Democrats in calling witnesses.
 
One Democrat, the centrist Sen. Joe Manchin of West Virginia, said he'd wouldn't have a problem hearing from Hunter Biden,who was on the board of a Ukrainian gas company, but doubted it will happen.
 
The witness swaps seem likely to fail as 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 is 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.
 
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.
 
Senators are to submit their written questions to be read by Chief Justice John Roberts. A key question for both sides is whether Trump's actions rise to the level of “high crimes and misdemeanors,” but that issue is seen through a highly partisan lens on each side.
 
               Trump faces charges from Democrats that he abused his power like no other president, jeopardizing U.S.-Ukraine relations by using the military aid as leverage while the vulnerable ally battled Russia. Democrats say Trump then obstructed their probe in a way that threatens the nation's three-branch system of checks and balances.
 
The president's legal team tried to lock up its case Tuesday and convince GOP senators that the president was right to ask Ukraine for investigations of Biden and his son Hunter and was well within his power to block the aid. They said he was not bound to abide by the congressional investigation.
 
Trump attorney Jay Sekulow addressed the Bolton controversy head-on in closing arguments by dismissing the former national security adviser's manuscript as “inadmissible." Attorney Alan Dershowitz, the Harvard scholar, said earlier that even if Bolton's story is true the actions don't rise to an impeachable offense.
 
Senate Republicans spent considerable time in private discussing how to deal with Bolton's manuscript without extending the proceedings or jeopardizing the president’s expected acquittal. They were set to meet privately again Wednesday.
 
Some Republicans including Sen. Pat Toomey want reciprocity -- bringing in Bolton or another Democratic witness in exchange for one from the GOP side. It's an idea initially pushed by conservative Sens. Josh Hawley, Ted Cruz and Mike Braun, separate from the centrists whose votes are in doubt. Some Republicans want to hear from Biden and his son, who was on the board of the Ukrainian gas company when his father was vice president.
 
“I don't know that the manuscript would make any difference in the outcome of the trial,” said Roy Blunt of Missouri, a member of GOP leadership. And some Republicans said they simply don’t trust Bolton’s word. Sen. Rand Paul of Kentucky called Bolton “disgruntled”’ and seeking to make money off his time at the White house.
 
But John Kelly, Trump's former White House chief of staff, told an audience in Sarasota, Florida, that he believes Bolton.
 
White House officials privately acknowledge that they are essentially powerless to block the book's publication but could sue after the fact if they believe it violated the confidentiality agreement Bolton signed.
 
Trump and his lawyers have argued repeatedly that Democrats are using impeachment to try to undo the results of the last presidential election and drive Trump from office.
 
Democrats, meanwhile, 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, according to a letter from Bolton's attorney.