Republican senators are sounding more optimistic they have the votes to soon end the impeachment trial of President Trump and shut down a wildcard phase of calling witnesses.

Despite Democrats’ drumbeat on the need to call witnesses like former National Security Adviser John Bolton, Republicans have signaled there’s growing momentum to shut down the trial sometime after the question phase concludes Thursday night -- though exactly how that plays out is an open question.

“I think, by that time, people will have heard enough,” Sen. John Barrasso, R-Wyo., told "Fox News @ Night." “They will have learned what they need to know about this case. And the plan is for Friday to vote. And the momentum is moving to say we've heard enough. Let's move to final judgment.”

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Sen. Mike Rounds, R-S.D., was optimistic impeachment could end as soon as late Friday if the witness vote fails and senators move to acquit the president.

“Obviously, I think we’re going to push to finish Friday,” Rounds said Wednesday night after the dinner break with his caucus.

Sen. John Thune, R-S.D., also cautiously upbeat, said if the witness votes fails, he’d like to see the Senate move swiftly toward an acquittal.

“My personal preference would be let's move quickly to close it out. But, that will probably be a group decision,” Thune said.

On Friday, senators will take the fateful vote on whether to continue the trial by subpoenaing witnesses and documents. After the trial begins at 1 p.m., the House managers and White House lawyers will have four hours, equally divided, to debate the merits of bringing Bolton and others to testify.

Democrats would need at least four Republicans to join with them to get 51 votes needed to pass.

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Sen. Dick Durbin, D-Ill., said he believes there are at least three Republicans who are seriously considering witnesses. “We need more,” he acknowledged Thursday.

GOP Sens. Mitt Romney of Utah, Susan Collins of Maine and Lisa Murkowski of Alaska are the three senators who have consistently signaled a willingness to join with Democrats.

"The fate of much of the future of how this republic conducts itself is on the shoulders of four Republicans," said Democratic Leader Chuck Schumer, D-N.Y.

"A trial without truth, without key evidence, without witnesses and documents would render the president's acquittal meaningless," Schumer added. "A giant asterisk next to it, because the trial was so rigged in his favor."

If the vote on witnesses fails Friday, there’s little else governing the next steps of the trial, except that the floor would be open to motions. Schumer declined to say what motions he’d seek, except that “the minority has rights and we’ll exercise those rights.”

Majority Leader Mitch McConnell will have first dibs on next steps and can put forth his initial proposal on how he’d like to end the trial. An up-or-down vote on acquittal could take place on Saturday.

Republicans hammered the case that if they open the door to witnesses, the Senate could become a circus. Deposing witnesses and potentially heading to court if Trump exerts executive privilege would just prolong the inevitable acquittal of the president, they argue. Sen. Lindsey Graham, R-S.C., has continually warned that if Democrats get Bolton, the GOP has the votes to haul in other witnesses Democrats don't want to see, including Hunter Biden.

Opening up the witness phase would be the like “wild west” with votes on who and what would be subpoenaed, Thune said.

“It's completely unknown territory. And there's a lot of uncertainty and unpredictability about what might happen next," he said.

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With swing state Sens. Cory Gardner of Colorado and Martha McSally of Arizona saying they are ready to shut down the trial, Republicans have been increasingly talking about ending the proceedings.

“It’s time to vote,” Sen. Josh Hawley, R-Mo., said.