(Bloomberg) -- Democrats face dwindling chances to get testimony from former National Security Adviser John Bolton and others in the Senate impeachment trial as the pool of Republicans willing to even consider defying President Donald Trump keeps shrinking.The possibility of new, potentially damaging revelations emerging from testimony or documentary evidence has always been the greatest unknown in a process where there’s little chance that two-thirds of the Republican-controlled chamber would vote to oust the president.It would take an extraordinary and unexpected effort by Republican senators to cross both Senate Majority Leader Mitch McConnell and Trump on the question of witnesses. They would risk withering criticism from their colleagues as well as Trump’s scorn, especially as he addresses them from the House rostrum Tuesday night during the State of the Union.Even Senate Minority Leader Chuck Schumer conceded on Wednesday that it will be a struggle for Democrats to prevent the trial from ending without witnesses testifying.”We’ve always known it will be an uphill fight on witnesses and on documents because the president and Mitch McConnell put huge pressure on these folks,” Schumer said during a break in the trial Wednesday, the first of two days that lawmakers had to question Trump’s defense and House prosecutors.Political ImpactGOP senators face cross pressures unlike any previous impeachment trial, since the president will be at the top of the ticket in the November election and damaging revelations could depress GOP turnout in Senate races.Republican senators Cory Gardner of Colorado and Pat Toomey of Pennsylvania, both of whom represent politically competitive states important in 2020 -- and were thought to be potential votes in favor of new witnesses -- indicated Wednesday they would vote against seeking additional evidence.Only Utah’s Mitt Romney said he firmly backed hearing from Bolton. Maine Senator Susan Collins, who like Gardner faces a tough re-election battle in November, signaled that she is likely to support calling witnesses. Republican Lisa Murkowski, another potential vote for testimony, refused to discuss her latest thinking after leaving a meeting with McConnell Wednesday morning.Democrats would need at least four Republicans to vote with them to open the impeachment trial to witnesses. Schumer said the public is “overwhelmingly on our side for witnesses,” and polls back him up. A Quinnipiac University released this week found three-quarters of U.S. voters say the Senate should hear from witnesses, in line with several other polls over the past month.McConnell has been working behind the scenes to shore up support for bringing the trial to a quick conclusion, after he told GOP senators in a closed-door meeting Tuesday he didn’t yet have the votes to block witnesses.The majority leader had to scramble after the bombshell disclosure that Bolton, who left the administration in September over policy disputes, wrote in a yet-to-be-published book that Trump linked aid for Ukraine to getting the country’s new president to announce a probe of former Vice President Joe Biden and his son Hunter Biden, who served on the board of a Ukrainian energy company.Bolton has said he would testify at the Senate trial if subpoenaed. The key votes will be held Friday, first on whether to allow additional testimony and then to select witnesses. A simple majority -- 51 senators -- will decide.Trump lawyer Jay Sekulow said there are witnesses he’d like to call, including House impeachment manager Adam Schiff, Joe and Hunter Biden and the intelligence community whistle-blower who triggered the House inquiry. But he warned that would create extended legal fights that would keep the trial in session for weeks, if not months.“If we get everybody we want we would be here for a very, very long time,” Sekulow said.Senator QuestionsThe trial entered a two-day phase on Wednesday for senators to submit questions to House prosecutors and Trump’s defense, an opportunity Democrats used to bolster their argument for more witnesses.Schumer asked House impeachment managers whether senators could render an accurate judgment without hearing from Bolton, acting White House Chief of Staff Mick Mulvaney and others.Schiff, who is leading the House impeachment managers, responded that they couldn’t, and he urged senators not to wait for Bolton’s book release to determine Trump’s real motives.”Don’t wait for the book,” Schiff said, referring to leaked revelations in Bolton’s manuscript. “Don’t wait until March 17 when it is in black and white to find out the answer to your question.”Trump’s lawyers sought to give wavering Republicans an off-ramp on the question of whether they needed to hear Bolton’s testimony. Collins, Murkowski and Romney jointly submitted a question to the defense asking how they should consider various motives by Trump for his actions. As Collins rose to ask their question, other senators paid keen attention.Trump lawyer Patrick Philbin answered that if there are mixed motives, for both policy and political reasons, the House’s case “fails and you can’t possibly have impeachment.” He also argued that investigating Biden’s role and that of his son were “in the interests of the United States” because the Ukrainian company, Burisma Holdings, was owned by an oligarch embroiled in corruption investigations.The same trio of senators submitted other questions together, and Trump lawyers were unable to answer questions regarding facts not already known, like when exactly did Trump order the aid withheld and what was his justification at the time? And did he ever discuss probing the Bidens before Joe Biden announced his run for president?Those questions could be addressed by some of the witnesses Democrats suggested calling to testify.‘Public Interest’Law professor Alan Dershowitz, speaking for Trump’s defense, asserted that a president’s power is expansive and he can’t be impeached for taking actions that are partly motivated by a desire to help his political prospects.“Every public official that I know believes that his election is in the public interest,” Dershowitz said in response to a question from Republican Senator Ted Cruz of Texas. “And if a president does something which he believes will help him get elected in the public interest, that cannot be the kind of quid pro quo that results in impeachment.”Schiff said that argument “would have terrified the founders” of the country. He said that would be like Congress saying “that a president can abuse their power in a corrupt way to help his re-election.”Dershowitz sought to walk back that comment Thursday morning with a tweet saying the media distorted his answer. “They characterized my argument as if I had said that if a president believes that his re-election was in the national interest, he can do anything,” Dershowitz tweeted. “I said nothing like that, as anyone who actually heard what I said can attest.”A number of Republican senators, including John Barrasso of Wyoming, have taken the position that Bolton’s account wouldn’t change their view of the impeachment charges.GOP Senator Josh Hawley of Missouri said Trump’s motives for asking for an investigation were well within his authority.“Presidents have the authority to do certain things. You’re not asking a foreign leader to engage in illegal conduct,” Hawley said.Three Democrats — Doug Jones, Joe Manchin and Kyrsten Sinema — haven’t said how they will vote on Trump’s ultimate conviction or acquittal.“I’m open to acquit. I’m open to convict,” Jones said. “I want to hear all the evidence. I want to hear witnesses.”Classified InformationMeanwhile, the White House worked to block or delay Bolton’s book, citing national security concerns.The National Security Council wrote to Bolton’s lawyer last week saying that his manuscript “appears to contain significant amounts of classified information” and can’t be published unless that material is deleted. The letter was obtained on Wednesday.A lawyer for Bolton said that he and his client “do not believe” that any of the material relating to Ukraine matters contained in a manuscript of his book “could reasonably be considered classified” -- and they told the White House that last week.Yet the lawyer, Charles Cooper, said there has been no response to that request sent on Friday for expedited pre-publication review in order to allow Bolton to testify about or publish the manuscript.Despite the lingering uncertainty on witnesses, South Dakota Republican Senator John Thune expressed confidence that the trial would get wrapped up quickly. He said Wednesday that GOP leaders will know where Republican votes are on the matter of more witnesses well before the tally is taken.North Dakota Republican Senator John Hoeven said he was confident enough that the Senate would finish with the trial by Friday that he booked airline tickets on Saturday as usual. Though he acknowledged things could change.”We have some tentative reservations like we always do,” he said. “That’s the Senate. Tentative reservations and you see how the votes go.”(Updates with Dershowitz tweet beginning in the 26th paragraph)\--With assistance from Daniel Flatley, Erik Wasson, Laura Davison and Billy House.To contact the reporters on this story: Mike Dorning in Washington at mdorning@bloomberg.net;Laura Litvan in Washington at llitvan@bloomberg.net;Steven T. Dennis in Washington at sdennis17@bloomberg.net;Laura Davison in Washington at ldavison4@bloomberg.netTo contact the editors responsible for this story: Joe Sobczyk at jsobczyk@bloomberg.net, Anna Edgerton, John HarneyFor more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.©2020 Bloomberg L.P.

Senators will resume asking questions of President Donald Trump’s defense and House prosecutors starting at 1 P.M. Thursday, marking the last chance for lawmakers to inquire about the president’s impeachment before moving to a vote on witnesses.

This story will continue to be updated. Keep scrolling for other recent highlights.

9:35 A.M.

Conservative group runs ad pressing Romney to reject witnesses

FreedomWorks is running a full-page ad against Sen. Mitt Romney to put new pressure on the Utah Republican to vote against witnesses.

The conservative group is running the ad online and in print in the Salt Lake Tribune. It seems unlikely to work; Romney is the most committed senator to hearing from former national security adviser John Bolton, whose unpublished book reportedly links Trump more directly with Ukraine aid freeze. — Burgess Everett

5:03 A.M.

How Trump's impeachment created two Democratic superstars

The House Democratic Caucus has long been dominated by a gaggle of also-rans: men and women who, while good enough for Congress, proved to be underwhelming on the larger stage of national politics.

That's changed.

Sitting shoulder to shoulder on the Senate floor as they argue for the president’s removal from office, two men — Adam Schiff of California and Hakeem Jeffries of New York — have been catapulted to the front of the nation's consciousness, to the top of the Democratic Party and have become the fulcrum for speculation about a host of prominent positions both in the House and beyond. Read the full story. — Jake Sherman and Heather Caygle

5:03 A.M.

Trump allies see witness-swap scheme as impeachment messaging coup

President Donald Trump’s allies are relishing the political benefits of a plot to offer an equal number of impeachment witnesses for both parties, even though it likely won’t go anywhere.

But that’s the point.

Republican strategists are viewing the offer as a potent tool in the PR war around the president’s impeachment trial. In their eyes, the proposal defangs a Democratic argument that Republicans are shutting down witnesses without actually having to call witnesses. It could give the appearance of fairness, they said, while also energizing the Trump base eager to at least imagine the president’s foes — like Joe Biden — taking the witness stand. Read the full story. — Anita Kumar

Sen. Elizabeth Warren’s Iowa campaign is getting off to a “ruff” start. Stuck in the nation’s capital for impeachment hearings, Warren (D-Mass.) has called on her pooch, Bailey, to campaign for her in the days ahead of the Iowa caucuses. Bailey has been canvassing the state with his dad and Warren’s husband, Bruce Mann, in...

Four GOP senators would have to join all the Democrats to ensure that witnesses are heard from.

The man who wrote "Impeach the President" almost 50 years ago still thinks we should impeach the president.