WASHINGTON — A divided Senate began the impeachment trial of President Trump on Tuesday in utter acrimony, as Republicans refused to commit to hearing from witnesses Democrats demanded and moderate Republicans forced last-minute changes to rules that had been tailored to the president’s wishes.
The debate, convened just after 1 p.m. in a Senate chamber transformed for the occasion, marked the substantive beginning of the third presidential impeachment trial in American history. It was the culmination of weeks of rhetorical sparring between the two parties over whether and when to call witnesses and new evidence that Mr. Trump blocked from House investigators.
Standing in the well of the Senate, the Democratic House impeachment managers pleaded with senators, who were sworn to silence, to reject proposed rules from the majority leader, Senator Mitch McConnell of Kentucky, that would delay a debate over witnesses and documents until the middle of the trial, with no guarantee that they would ever be called.
The question of calling new evidence, said Representative Adam B. Schiff of California, the lead manager, was an “even more important question than how you vote on guilt or innocence.”
“If the Senate votes to deprive itself of witnesses and documents, the opening statements will be the end of the trial,” he said. “To say let’s just have the opening statements and then we’ll see means let’s have the trial, and maybe we can just sweep this all under the rug.”
Senate Democrats planned a series of amendments for later in the day to try to commit the chamber to subpoenaing four specific witnesses and tranches of documents. With Republicans in firm control of their 53-to-47 majority, each was expected to fail. But the issue of witnesses is expected to resurface later in the trial, after opening arguments and a question period, when the rules allow votes on whether and whom to subpoena.
At the heart of the trial are charges of abuse of power and obstruction of Congress against the president approved last month nearly along party lines by the Democratic-led House. They assert that Mr. Trump used the power of his office to enlist a foreign power for help in the 2020 election by pressuring Ukraine to announce investigations of his political rivals while withholding as leverage nearly $400 million in military aid and a White House meeting for its president, and then stonewalled congressional attempts to investigate.
Mr. Trump’s legal team argues that the charges are baseless and amount to criminalizing a president’s prerogative to make foreign policy decisions as he sees fit and to shield from Congress documents relating to the conduct of his duties. They also claim — in a break with most constitutional scholars — that because the articles of impeachment do not outline a specific violation of a law, Mr. Trump cannot be impeached.
But on Tuesday, the debate focused on whether his trial would be fair or not.
“This initial step will offer an early signal to our country: Can the Senate still serve our founding purpose?” Mr. McConnell said.
But under pressure from Republican moderates, Mr. McConnell made some last-minute changes to the set of rules he unveiled on Monday, which would have squeezed opening arguments by both sides into two 12-hour marathon days and refused to admit the findings of the House impeachment inquiry into evidence without a separate vote later in the trial.
Senators Susan Collins of Maine and Rob Portman of Ohio, among others, objected to those proposals, which were a departure from procedures adopted unanimously by the Senate for the 1999 trial of President Bill Clinton, on which Mr. McConnell had promised to model his rules. At a luncheon with Republican senators in the Capitol just before the trial was to begin, Ms. Collins and Mr. Portman raised their objections privately, according to aides familiar with the conversation, and Mr. McConnell submitted a revised copy of the resolution — with lines crossed out and changes scrawled in the margins — when it was time for the debate.
When his resolution was read aloud on the Senate floor, two days had been extended to three and the House’s records would be automatically admitted into evidence, although Mr. McConnell inserted a provision not included in the 1999 rules that would allow Mr. Trump’s team to move to throw out parts of the House case.
The historically rare debate was rendered even more unusual by Senate rules that prohibit senators from speaking on the chamber floor for the duration of the proceedings and empower the House managers and White House defense lawyers to argue aloud over the proposals instead. The effect was that on the trial’s first day, the Senate chamber split cleanly into partisan factions, with the managers siding with Senate Democrats and Mr. Trump’s lawyers taking the place of the Republicans.
Pat A. Cipollone, the White House counsel, rose first and addressed the senators, urging them to support Mr. McConnell’s proposed rules, which he called “a fair way to proceed” and getting in a jab at House Democrats’ delay in transmitting the articles of impeachment to the Senate.
“We believe that once you hear those initial presentations, the only conclusion will be that the president has done nothing wrong,” Mr. Cipollone said, “and that these articles of impeachment do not begin to approach the standard required by the Constitution.”
Locked in silence, senators were able to speak only before the proceeding began or during brief breaks. Speaking to reporters on Tuesday morning, Senator Chuck Schumer of New York, the Democratic leader, pledged to force votes later in the day on about a half-dozen amendments that would, if adopted, direct the Senate to subpoena documents and witnesses that evaded House investigators.
“It is completely partisan. It was kept secret until the eve of the trial,” he said. “The McConnell rules seem to have been designed by President Trump and for President Trump, simply executed by Leader McConnell and Senate Republicans.”
Specifically, he planned to force votes on records housed at the White House, State Department and Defense Department, and to compel testimony from four current and former Trump administration officials who were blocked from speaking with the House: John R. Bolton, the former White House national security adviser; Mick Mulvaney, the acting White House chief of staff; Robert B. Blair, an adviser to Mr. Mulvaney; and Michael Duffey, a White House budget official.
Mr. McConnell promised to move to table each amendment so it could not be adopted, but the debate could take hours.
“This is the fair road map for our trial,” he said in a speech on the Senate floor not long after Mr. Schumer’s remarks. “We need it in place before we can move forward. So the Senate should prepare to remain in session today until we complete this resolution and adopt it.”
Even after Tuesday’s changes, Mr. McConnell’s proposal makes way for potentially the fastest presidential impeachment trial in American history, particularly if the Senate declines to call witnesses.
Only two other American presidents have stood trial in the Senate for high crimes and misdemeanors. Andrew Johnson was impeached in 1868, and his trial took the better part of three months, featuring testimony from dozens of witnesses and extended periods for discovery, before he was ultimately acquitted by just a single vote. Mr. Clinton was tried in 1999 on charges of perjury and obstruction of justice. That proceeding lasted five weeks, included testimony from just three witnesses and resulted in an overwhelming acquittal.
Without witnesses, Mr. Trump’s trial could conclude by the end of January. If senators ultimately do call witnesses, that timeline could stretch weeks longer.
Just an hour or so before the trial began, the seven House managers submitted one final written rebuttal to arguments put forward against their charges by Mr. Trump’s lawyers. In 34 detailed pages, they rejected the lawyers’ assertion that abuse of power was not an impeachable offense and that Mr. Trump had acted legally when he ordered administration officials not to appear for questioning in the House or provide documents for the impeachment inquiry.
Catie Edmondson and Emily Cochrane contributed reporting.