WASHINGTON — Key Republican senators who could tip President Donald Trump’s impeachment trial toward summoning more witnesses played an over-sized role in the final hours of debate Thursday night with pointed questions ahead of crucial votes.

A vote on witnesses, expected Friday, could lead to an abrupt end of the trial with the expected acquittal. Or it could bring days, if not weeks more argument as Democrats press to hear testimony from former national security adviser John Bolton and others.

Any four Republicans could join with Democrats to demand taking more time for testimony.

Sen. Lisa Murkowksi of Alaska drew a reaction when she asked simply: “Why should this body not call Ambassador Bolton?”

GOP Sen. Lamar Alexander of Tennessee captured attention just before the dinner break when he questioned partisanship in the proceedings thus far. A spokesman confirmed to The Associated Press that Alexander would announce his decision on the witness vote shortly after the end of Thursday’s questions.

Sen. Susan Collins, the Maine Republican whose vote on witnesses was considered in the balance, wanted to know why House Democrats withdrew a subpoena for a deputy national security adviser they wanted to hear from in the impeachment inquiry.

In response to Alexander and others, Democrat Rep. Zoe Lofgren of California, a congressional staffer during Watergate and now a House prosecutor, told the senators that the Nixon impeachment also started as a partisan inquiry. A bipartisan consensus emerged only after Republicans — including staunch Nixon supporters — saw enough evidence to change their minds, she said.

“They couldn’t turn away from the evidence that their president had committed abuse of power and they had to vote to impeach him,’’ Lofgren said. Richard Nixon resigned before he was impeached.

While disappointed that House Republicans did not join Democrats in voting to impeach Trump, she said the Senate — “the greatest deliberative body on the planet’’ — has a new opportunity.

Alexander, after his question Thursday night, consulted with a key staff aide to Majority Leader Mitch McConnell. As the senators broke for dinner Alexander and Murkowski met privately.

Republican Sen. Mitt Romney of Utah is also among those being closely watched.

Trump was impeached by House last month on charges that he abused his power like no other president, jeopardizing Ukraine and U.S.-Ukraine relations. Democrats say Trump asked he vulnerable ally to investigate Joe Biden and debunked theories of 2016 election interference, temporarily halting American security aid to the country as it battled Russia at its border. 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.

Thursday’s testimony included soaring pleas to the senators-as-jurors who will decide Trump’s fate, to either stop a president who Democrats say has tried to cheat in the upcoming election and will again, or to shut down impeachment proceedings that Republicans insist were never more than a partisan attack.

“Let’s give the country a trial they can be proud of,” said Rep. Adam Schiff, the lead prosecutor for House Democrats. Americans, he said, know what it takes for a fair trial. He offered to take just one week for depositions of new witnesses, sparking new discussions.

Trump attorney Eric Herschmann declared the Democrats are only prosecuting the president because they can’t beat him in 2020.

“We trust the American people to decide who should be our president,” Herschmann said. “Enough is Enough. Stop all of this.”

McConnell was toiling to keep Friday’s vote on schedule even as the trial was unearthing fresh evidence from Bolton’s new book and raising alarms among Democrats and some Republicans about a Trump attorney’s controversial defense.

In a day-after tweet, Trump attorney Alan Dershowitz, complained about the portrayal of his Wednesday night testimony when he said a president is essentially immune from impeachment if he believes his actions to be in the “national interest.”

That idea frustrated some inside the White House, who felt Dershowitz’s claim was unnecessary and inflammatory — irking senators with a controversial claim of vast executive powers. But those officials left it to Dershowitz to back away, wary that any public White House retreat would be viewed poorly by the president.

“I said nothing like that,” the retired professor tweeted Thursday.

His words Wednesday night: “Every public official that I know believes that his election is in the public interest. And if a president does something which he believes will help him get elected is in the public interest, that cannot be the kind of quid pro quo that results in impeachment.”

Asked about it as one of the first questions Thursday, Democrat Schiff, said, “Have we learned nothing in the last half century?”

Schiff drew on the lessons of the Nixon era to warn of a “normalization of lawlessness” in the Trump presidency.

“That argument — if the president says it it can’t be illegal — failed when Richard Nixon was forced to resign,” Schiff told the senators. “But that argument may succeed here, now.”

“This is not a banana republic,” said Rep. Hakeem Jeffries, D-N.Y., rejecting the White House counsel’s suggestion there was nothing wrong with seeking foreign election interference.

Democrats played a video showing the many times Trump has campaign.

The president has argued repeatedly that his dealings with Ukraine have been “perfect.”

Republicans Collins, Romney and Murkowski all have expressed interest in hearing from Bolton and the others.

In a Senate split 53-47 with a Republican majority, 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.

Chief Justice John Roberts, presiding over the chamber and fielding senators’ questions for the trial, could break a tie, but that seems unlikely.

The chief justice did exercise authority Thursday with a stunning rebuttal to a question posed by Sen. Rand Paul of Kentucky designed to expose those familiar with the still anonymous whistleblower whose complaint about Trump’s phone call with Ukraine’s new president led to the impeachment inquiry.

Roberts had communicated through his staff to McConnell’s office that he did not want to read the whistleblower’s name, according to a Republican unauthorized to discuss the private conversation and granted anonymity.

“The presiding officer declines to read the question as submitted,” he said.

Senators have dispatched with more than 100 queries over two days. The questions came from the parties’ leaders, the senators running for the Democratic nomination against Trump and even bipartisan coalitions from both sides of the aisle.

Trump’s team says the House’s 28,000-page case against the president and the 17 witnesses — current and former national security officials, ambassadors and others who testified in the House proceedings — are sufficient.

Instead, Trump’s lawyers focused some of their time Thursday refloating allegations against Biden and his son, Hunter Biden, who served on the board of a gas company in Ukraine while his father was vice president.

Rep. Val Demings, D-Fla., responding to one question said the Bidens have little to tell the Senate about Trump’s efforts to “shake down” Ukraine for his own campaign.

Democrats argued Bolton’s forthcoming book cannot be ignored. It contends he personally heard Trump say he wanted military aid withheld from Ukraine until it agreed to investigate the Bidens — the abuse of power charge that is the first article of impeachment. Trump denies saying such a thing.

The White House has blocked its officials from testifying in the proceedings and objected in a letter to Bolton’s attorney to “significant amounts of classified information” in the manuscript, including at the top secret level. Bolton resigned last September — Trump says he was fired — and he and his attorney have insisted the book does not contain any classified information.