WASHINGTON – Two pivotal Senate Republicans – Susan Collins of Maine and Mitt Romney of Utah – announced Thursday they would vote to subpoena witnesses in the impeachment trial of President Donald Trump, but it remained unclear whether there were enough votes for additional testimony.

Both were expected to join Democrats in voting to subpoena witnesses such as former national security adviser John Bolton, along with potential swing votes from GOP Sens. Lamar Alexander of Tennessee and Lisa Murkowski of Alaska

But Alexander said he would vote against witnesses.

“I worked with other senators to make sure that we have the right to ask for more documents and witnesses, but there is no need for more evidence to prove something that has already been proven and that does not meet the United States Constitution’s high bar for an impeachable offense," Alexander said.

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The announcements turned all eyes on Murkowski, who told reporters she is still weighing her options.

“I am going to go reflect on what I have heard, re-read my notes and decide whether I need to hear more," she said.

Because a majority of the Senate sets policy in the trial, the 47 Senate Democrats need at least four of the 53 Republicans to support subpoenas to summon witnesses or documents, as House managers have urged.

The Senate is expected to vote Friday on subpoenas. A motion to subpoena a witness would fail on a 50-50 tie. It's not clear whether Supreme Court Chief Justice John Roberts, who is presiding, would break the tie.

Alexander, Collins, Murkowski and Romney were among a handful of Republican senators who kept open the possibility of calling witnesses as House Democrats, who prosecuted the case, and Trump's defense team each presented three days of opening arguments. Senators posed written questions for both sides during eight-sessions Wednesday and Thursday.

Bolton's offer became more urgent after the New York Times reported that his pending book described Trump telling his aide that he suspended $391 million in military assistance to Ukraine under the condition of investigations of former Vice President Joe Biden and his son, Hunter.

But Trump has denied telling Bolton that the Ukraine aid was tied to investigations. He also suggested he might fight to block Bolton's testimony under executive privilege, to protect the confidentiality of advice from top aides.

Collins said she would support witnesses and said that if they are approved, House managers and Trump's defense team should agree on a limited number for each side. The Senate could then vote on the precise number, she said.

"I believe hearing from certain witnesses would give each side the opportunity to more fully and fairly make their case, resolve any ambiguities, and provide additional clarity," Collins said. "Therefore, I will vote in support of the motion to allow witnesses and documents to be subpoenaed."

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Murkowski offered a hint about her thinking when she asked Trump's defense team Thursday why the Senate shouldn't call Bolton.

"This dispute about material facts weighs in favor of calling additional witnesses with direct knowledge," Murkowski said in the written question, read by Supreme Court Chief Justice John Roberts.

Patrick Philbin, a deputy White House counsel, warned that agreeing to call additional witnesses would establish new standards for impeachment, if the House can send "half-baked" cases and leave the Senate to complete the investigation. Philbin also said Bolton hadn't confirmed the newspaper report.

“It will do grave damage to this body as an institution to say that the proceedings in the House don’t have to really be complete," Philbin said. “That’s not the way the way that his chamber should allow impeachments to be presented to it."

The clash over witnesses has been contentious throughout the trial. As the trial opened, the Senate rejected Democratic proposals for witnesses in a series of party-line votes, but held out the prospect for more votes after arguments and questions were done.

House managers led by Intelligence Chairman Adam Schiff, D-Calif., said the trial wouldn't be fair without compelling witnesses who refused to testify during the impeachment inquiry, including Bolton and acting White House chief of staff Mick Mulvaney.

Schiff argued that the House couldn't wait for courts to enforce subpoenas because an appeals decision is still pending for former White House counsel Don McGahn, while Trump represents a threat to the 2020 election.

“He will not be acquitted,” House Speaker Nancy Pelosi, D-Calif., told reporters Thursday about Trump. “You cannot be acquitted if you don’t have a trial. And you don’t have a trial if you don’t have witnesses and documentation.”

But Trump’s defense team said the House should have summoned witnesses and fought in court to enforce subpoenas. Jay Sekulow, another defense lawyer, argued that no witnesses were needed because the House managers said they already had overwhelming evidence to prove their case.

But Sekulow said Trump's team would want to call its own witnesses if any were allowed. Sekulow said he would like to call Joe Biden, Hunter Biden and the whistleblower, whose complaint about Trump's dealings with Ukraine sparked the impeachment inquiry.

Trump's lawyers said the trial could last months longer if senators open the door to witnesses. But Schiff suggested that witnesses could be deposed within a week and that Supreme Court Chief Justice John Roberts, who is presiding over the trial, could quickly resolve disputes over which witnesses are relevant and which deserve executive privilege.

Contributing: Nicholas Wu, Savannah Behrmann