WASHINGTON — President Trump’s defense team and the Democratic prosecutors clashed on Thursday over the motives behind his pressure campaign on Ukraine and the consequences of removing him from office, as the Senate moved toward a vote on whether to summon witnesses to his impeachment trial or speed to a likely acquittal.
On the second consecutive day of questioning from senators, each side sought to appeal to a shrinking group of undecided Republicans who on Friday must choose to stick with their party and block additional witnesses and documents, bringing a swift end to the trial, or break with Mr. Trump and demand more evidence.
As the trial neared a critical turning point, with senators about to weigh the removal of a president for the third time in American history, both parties were looking beyond the verdict and framing their arguments to voters ahead of the November elections.
“If the American people decide that they don’t like what’s happened here, that they don’t like the constitutional violations that have happened, that they don’t like the attack on a successful president for purely partisan political purposes, then they can do something about it, and they can throw them out,” said Pat A. Cipollone, the White House counsel.
Bracing for a possible defeat of their efforts to force witnesses to be heard at the trial, Democrats forecast what is likely to be their message after the verdict is reached, asserting that Mr. Trump’s acquittal would be illegitimate because the trial was flawed.
“He will not be acquitted,” Speaker Nancy Pelosi, Democrat of California, told reporters at her weekly news conference. “You cannot be acquitted if you don’t have a trial. You don’t have a trial if you don’t have witnesses and documentation and that.”
Senate Republican leaders projected confidence that they would line up the requisite votes, and privately plotted the trial’s endgame. But even as Republicans in politically competitive states pulled together in opposition to witnesses, party leaders could not yet say definitively that they would block the additional evidence.
“We are getting to where we need to be on the witness vote,” said Senator Roy Blunt, Republican of Missouri and a member of his party’s leadership.
Senator Mitch McConnell, Republican of Kentucky and the majority leader, toiled behind the scenes to cajole wavering moderates and stifle any move to prolong the trial by admitting additional evidence.
Asked about the looming vote as he arrived on Thursday morning to the Capitol, he told reporters: “I’m always confident.”
If Democrats remain united, which is not guaranteed, they would need four Republicans to join them to support calling witnesses. Senators Susan Collins of Maine and Mitt Romney of Utah have already indicated they are likely to vote with Democrats, leaving the decisive vote to two other Republican moderates, Lisa Murkowski of Alaska and Lamar Alexander of Tennessee.
Ms. Murkowski appeared to be grappling with her position on Thursday evening. After the Senate broke for dinner, she submitted a question for Mr. Trump’s defense team asking why the Senate should not hear from John R. Bolton, his former national security adviser, who wrote in an unpublished book that Mr. Trump said he would not release the military aid for Ukraine until the country assisted in investigations of his political rivals.
She noted that account, reported on Sunday by The New York Times, contradicted Mr. Trump’s explicit denials that he had linked the military aid and investigations.
“This dispute about material facts weighs in favor of calling additional witnesses with direct knowledge,” Ms. Murkowski asked. “Why should this body not call Ambassador Bolton?”
Mr. Alexander indicated that he would announce his position on witnesses when the questioning concluded on Thursday night.
Earlier inside the Senate chamber, the usually scripted trial got off to a tense start on Thursday when Senator Rand Paul, Republican of Kentucky, submitted a question that included the name of a person widely believed to be the C.I.A. whistle-blower whose anonymous complaint about Ukraine helped prompt the impeachment inquiry.
After studying the question card, Chief Justice John G. Roberts Jr. refused to read it aloud, instead saying, “The presiding officer declines to read the question as submitted.”
An indignant Mr. Paul then rushed out of the chamber straight to waiting television cameras to read the question aloud himself. He asked if the prosecution and defense teams were aware of reports that two government officials “may have worked together to plot impeaching the president before there were formal House impeachment proceedings.”
Mr. Paul’s move reflected how Mr. Trump’s conservative allies have sought to turn the tables on the entire inquiry, shifting the focus away from the president’s conduct and toward what they suggest was a conspiracy by his opponents to manufacture a basis for removing him.
The theme has been a favorite of the president’s, and is all but certain to figure prominently in his re-election campaign after his acquittal as he makes the case to voters that the impeachment effort was an affront to them as much as a challenge to him.
Senators spent much of the day posing dozens of leading questions and pointed challenges to the House managers and Mr. Trump’s defense team, many of them intended to relitigate arguments offered earlier in the trial or embarrass the opposing party.
All but a handful of Republicans appeared ready to dispense with the abuse of power and obstruction of Congress charges and vote to acquit on both counts. The same proportion of Democrats signaled that they were receptive to the managers’ charges.
In a rare bipartisan question — one of only three out of more than 150 submitted — Ms. Murkowski and Senator Brian Schatz, Democrat of Hawaii, asked that given the actions of any president are to some extent inherently political, how should senators distinguish between permissible political actions and impeachable ones.
Patrick Philbin, a deputy White House counsel, said trying to discern a politician’s motive “is very dangerous.”
“There is always some eye to the next election,” he said, “and it ends up becoming a standard so malleable that in reality really is a substitute for a policy difference: If we don’t like a policy difference, we attribute it to a bad motive.”
Representative Adam B. Schiff, Democrat of California, countered that impeachment was the appropriate “political punishment for a political crime” involving corrupt activity.
“If we go down that road” of ignoring a corrupt motive, Mr. Schiff said, “there is no limit to what this or any other president can do.”
The question touched on a larger debate hanging over the trial since Sunday, when The Times reported on Mr. Bolton’s conversation with Mr. Trump.
Senators are wrestling with whether to call Mr. Bolton to testify, but his account has already functionally knocked out one of the president’s chief defenses: that the press for investigations and the aid freeze were unconnected.
In its absence, the debate has shifted to whether such actions would even be impeachable, and if so, whether Mr. Trump’s motivations were corrupt when he sought the investigations.
But even as Republican senators ceded many of the facts of the House case were likely true, Mr. Trump’s legal team walked a fine line.
“We have gone through the evidence that has made it quite clear that, both with respect to a bilateral meeting with the president and a temporary pause on the security assistance, the evidence just doesn’t stack up to show President Trump linked them here,” Mr. Philbin said.
At one point, Eric Herschmann, one of Mr. Trump’s lawyers, implored senators to consider the president’s record of accomplishments — launching into a long, campaign-style litany of trade pacts sealed, unemployment rates falling and miles of wall built on the southwestern border — effectively arguing that those factors should offset the charges against him.
“If all that is solely — solely, in their words — for his personal and political gain and not in the best interests of the American people, then I say God bless him — keep doing it,” Mr. Herschmann said. “Maybe if the House managers stop opposing him, and harassing him, and harassing everyone associated with him with the constant letters and the constant investigations, maybe we can even get more done.”
“Enough is enough,” he added. “Stop all of this.”
The vote on Friday, which will most likely take place sometime in the late afternoon, does not deal directly with individual witnesses or sets of documents. Rather, it will resolve whether the trial should even consider calling additional witnesses and evidence.
In a bid to reassure moderate Republicans concerned that voting for witnesses would prolong the trial indefinitely, Mr. Schiff proposed on Thursday that the Senate agree to a contained, one-week period in which it would conduct depositions of witnesses proposed by the prosecution and defense. Any dispute about the relevance of witnesses or executive privilege, he suggested, could be settled by the chief justice.
If a majority of senators vote no, Republican leaders could move the proceeding to final deliberations and a speedy up or down vote on each article of impeachment, possibly as early as Friday. If they vote yes, the trial would blow open and could become a free-for-all in which any group of 51 senators could band together to issue subpoenas for testimony and records of their choosing.
But senators in both parties were also bracing for the distinct possibility that the witness and documents vote could end in a 50-50 tie. Such an outcome would put Chief Justice Roberts in a difficult position. There is precedent, drawn from the 1868 impeachment trial of President Andrew Johnson, of a chief justice casting a tiebreaking vote on a procedural motion. But despite Democratic hopes, Chief Justice Roberts is unlikely to want entangle himself in a dispute that has been so thoroughly politicized.
If the chief justice abstained from breaking a tie, the motion would fail.
Republican leaders believe they will need to accommodate any Democratic motions or amendments to prolong the trial, as well as closing arguments from the House managers and presidential defense team. It was uncertain whether senators would demand time to deliberate over their final votes in a closed session, as was done over three days during the 1999 trial of President Bill Clinton.
“The goal would be to get this done tomorrow evening,” Senator John Barrasso of Wyoming, the No. 3 Republican in the Senate, told reporters before the trial began for the day.
But other Republicans involved in the discussions cautioned Mr. Barrasso’s assessment was most likely overly optimistic, and that final votes could slip into Saturday or even Monday.
“The minority has rights, and we will exercise those rights,” said Senator Chuck Schumer of New York, the Democratic leader.
Michael D. Shear, Sheryl Gay Stolberg and Catie Edmondson contributed reporting.