WASHINGTON — Senate leaders wrangled on Friday over the terms of President Trump’s impeachment trial, deepening their impasse and throwing the start date further into doubt amid disputes over whether to call witnesses or introduce documentary evidence.
In a pair of barbed back-to-back speeches on the Senate floor, the top Republican and the top Democrat traded charges of hypocrisy and unfair dealing, settling nothing in the high-stakes fight over the shape of the constitutional proceeding that will decide Mr. Trump’s fate.
The feuding on the first day of a new legislative session indicated that two weeks of holiday respite had done little to break a logjam. It began late last year, after Speaker Nancy Pelosi said she would not transmit two impeachment articles approved by the Democratic-led House to prompt the start of a trial until the Senate pledged a fair process.
“Their turn is over,” Senator Mitch McConnell, Republican of Kentucky and the majority leader, said Friday of House Democrats. “They have done enough damage. It is the Senate’s turn now to render sober judgment as the framers envisioned.”
With no meeting between the Senate leaders scheduled, nor any indication of when Ms. Pelosi might send the articles, lawmakers and aides privately warned that the stalemate could stretch on into mid-January, with Mr. Trump’s presidency hanging in the balance.
Mr. McConnell, conceding that his chamber could not act without the House acting first, said Republicans would simply turn back to their regular legislative work while they waited, “content to continue the ordinary business of the Senate while House Democrats continue to flounder.”
Stepping onto the Senate floor moments after Mr. McConnell, Senator Chuck Schumer of New York, the Democratic leader, accused the president’s party of trying to steamroller to his acquittal.
“We are not asking for critics of the president to serve as witnesses in the trial,” Mr. Schumer said. “We are asking only that the president’s men, his top advisers, tell their side of the story. And Leader McConnell has been unable to make one argument — one single argument — as to why these witnesses and these documents should not be part of a trial.”
The outcome of the dispute over witnesses and documents could determine not only when the trial begins and how long it lasts, but whether it produces meaningful new information that could affect the outcome, either exonerating or further implicating Mr. Trump.
The House impeached the president after an inquiry that exposed how he pressed Ukraine’s president to investigate Mr. Trump’s political rivals, including former Vice President Joseph R. Biden Jr., while withholding nearly $400 million in military assistance that Kyiv needed to beat back lethal Russian aggression. Democrats charged that the conduct amounted to an effort by the president to enlist a foreign power to help him in the 2020 election, using the tools of United States foreign policy and American taxpayer dollars as leverage.
Republicans contended that Mr. Trump was merely trying to root out corruption in a former Soviet republic that had long been known for it, though they contested scant few of the facts drawn out by weeks of testimony in the House.
Democrats have argued that in light of Mr. Trump’s blanket defiance of the House’s impeachment inquiry, the trial must include new evidence to be fair, and they want a commitment up front.
Mr. McConnell vehemently disagrees, though on Friday he did not rule out calling witnesses. He asked instead for the Senate to follow the precedent of the 1999 impeachment trial of President Bill Clinton, when lawmakers agreed unanimously to open the proceedings without an agreement on whether to call witnesses, and decided on that issue only after both sides presented opening arguments.
Mr. Schumer has asked that a trial include four witnesses and a wide array of documents that the White House blocked House investigators from accessing. His case has been aided in recent days as new evidence has surfaced in news reports and freedom of information lawsuits.
On Friday, as Mr. Schumer was addressing the Senate, a federal judge in New York ruled that Lev Parnas, an associate of Rudolph W. Giuliani’s involved in elements of the Ukraine scheme, could begin sharing additional documents and communications with the House.
“Each new revelation mounts additional pressure on the members of this chamber to seek the whole truth,” Mr. Schumer said.
Negotiations may become only more complicated in the days ahead, as Washington also contends with the aftermath of an early-Friday American drone strike that killed Iran’s top security and intelligence commander. The commander, Maj. Gen. Qassim Suleimani, led the powerful Quds Force of the Islamic Revolutionary Guards Corps, and with Iran already promising retaliation, the action ordered by Mr. Trump threatened to escalate a dangerous conflict in the region.
The drone strike at Baghdad International Airport drew comparisons to Mr. Clinton’s four-day bombing campaign of Iraq in December 1998 as he stared down his own impeachment. At the time, some Republicans accused the president of using a military incursion to distract from his domestic political problems.
Mr. Trump has made little secret of his eagerness for the Republican-controlled Senate to mount an elaborate trial that would not only dispense with the charges, but also provide an opportunity for his allies to defend him, and Ms. Pelosi believes that withholding the impeachment charges could provide leverage in shaping the trial before it begins.
In his floor speech on Friday, Mr. McConnell emphatically rejected that strategy. He denounced Democrats in both the House and Senate for degrading the impeachment process with what he called a historically weak case. Calling additional witnesses during a Senate trial, as Democrats have demanded, would not change that, he said.
“The same people who just spent weeks screaming that impeachment was so serious and so urgent that it couldn’t wait for due process now decided it would wait indefinitely while they checked the political whims and looked for new talking points,” Mr. McConnell said.
While Mr. Trump has repeatedly claimed that he was being denied due process rights in the House impeachment proceeding, the White House refused to participate when offered the opportunity.
Mr. McConnell vociferously defended himself against Democratic charges that he was unfairly prejudging the case and sought to turn the tables on Democrats, who he said were “openly coordinating political strategy with the speaker, who some might call the prosecution.”
With Mr. McConnell firmly rooted in his position, Mr. Schumer appealed to moderate Republican senators to ensure the Senate had a “fair trial.” If Democrats remain united, they need only four Republicans to temporarily join them to command a majority able to set the rules of the trial.
“If we don’t get a commitment up front hat the House managers will be able to call witnesses as part of their case then the Senate will act as little more than a nationally televised meeting of the mock trial club,” Mr. Schumer said.
Republicans hope to make Democrats pay politically for the delay.
Senator Josh Hawley, Republican of Missouri, said he planned to introduce a resolution on Monday that would amend the Senate impeachment rules to allow the chamber to dismiss the House’s charges if Democrats did not promptly bring them to trial. The resolution’s constitutionality is questionable, and it will almost certainly not receive a vote, but it drew attention to Republicans’ argument that Democrats were acting out of political interest.
“This will expose Dems’ circus for what it is: a fake impeachment, abuse of the Constitution, based on no evidence,” Mr. Hawley, a first-term lawmaker, wrote on Twitter. “If Dems won’t proceed with trial, bogus articles should be dismissed and @realDonaldTrump fully cleared.”
Mr. Schumer has his own backup. Senator Dianne Feinstein of California, the top Democrat on the Judiciary Committee, circulated a letter to colleagues on Thursday asking them to support a motion formally requesting testimony from the four officials and the production of relevant documentary records.
“Agreement on Senator Schumer’s requests for these witnesses and documents requires a simple majority,” she wrote. “This should be easy to achieve as all Senators should want this information from the outset to ensure a full and fair trial.”
The witnesses Democrats wish to see testify include John R. Bolton, Mr. Trump’s former national security adviser; Mick Mulvaney, the acting White House chief of staff; Robert B. Blair, Mr. Mulvaney’s senior adviser; and Michael Duffey, a White House budget aide.