WASHINGTON — With the House headed to a vote to impeach President Trump next week, Senator Mitch McConnell was working hand in hand with the White House to make plans for a Senate trial, a proceeding steeped in tradition and rules but one fraught with political peril for vulnerable Republicans.
Mr. Trump said on Friday that he had no preference for how the trial — expected to begin in early January — unfolds, but he has privately pushed for a prolonged process that would allow him to mount a theatrical defense. Mr. McConnell, Republican of Kentucky and the majority leader, has resisted that idea in favor of a shorter, more dignified event.
Outraged Democrats, meanwhile, accused Mr. McConnell on Friday of abandoning his duty to render “impartial justice” in an impeachment trial — a response to a television interview in which Mr. McConnell dismissed House Democrats’ articles of impeachment as “so darn weak.” He added that he was “taking my cues” from the White House in shaping the trial.
“If articles of impeachment are sent to the Senate, every single senator will take an oath to render ‘impartial justice,’” Senator Chuck Schumer of New York, the Democratic leader, said Friday in a statement, in a veiled reference to those remarks. “Making sure the Senate conducts a fair and honest trial that allows all the facts to come out is paramount.”
Representative Val B. Demings, Democrat of Florida and a member of the House Judiciary Committee who is being mentioned as a possible House manager during the impeachment trial, went one step further, and called Friday for Mr. McConnell to recuse himself from the proceedings.
“No court in the country would allow a member of the jury to also serve as the accused’s defense attorney,” Ms. Demings said in a statement. “The moment Senator McConnell takes the oath of impartiality required by the Constitution, he will be in violation of that oath.”
Mr. McConnell and White House officials — including Pat A. Cipollone, the White House counsel, and Eric Ueland, the White House congressional liaison — have been meeting privately to hash out their differences and present a plan for the trial to Mr. Trump, but no agreements have been reached, according to two people close to the talks.
Mr. Cipollone is expected to represent Mr. Trump at the trial, along with the president’s outside lawyers. But the president has also been quizzing people about who his lawyers should be, and has noted Mr. Cipollone’s lack of TV experience, as the trial will be televised, a person involved in the planning said.
The negotiations are likely to continue through the weekend, as the House prepares for what is almost certain to be a party-line vote to impeach the president in connection with his campaign to enlist Ukraine to investigate former Vice President Joseph R. Biden Jr. and Mr. Biden’s son Hunter Biden.
The talks are complicated, with a lot of variables: how long the trial will last; how much time each side will get to present its case; whether any witnesses will be called, and if so, how many; how documentary evidence will be handled; and so on. There are questions about who will get to speak and where people will sit.
But the central question appears to be whether witnesses will testify. Mr. Trump has said he wants to call Hunter Biden, as well as Speaker Nancy Pelosi and Representative Adam B. Schiff, the chairman of the House Intelligence Committee, who led the fact-finding phase of the impeachment inquiry.
The McConnell camp worries that could open a “Pandora’s box,” as one person close to the senator said, clearing the way for other witnesses and lines of questioning that could reflect poorly on the president. But some House Republicans say it is important to do so.
“I understand their desire to just get it behind us, but the country needs to hear what a farce this was,” Representative Louie Gohmert, Republican of Texas and a member of the House Judiciary Committee, said Friday. “They really need to bring in witnesses. They are the chance to clean this mess up.”
The Senate has specific rules, revised in 1986, governing impeachment trials. Among other things, once the House adopts articles of impeachment and presents them to the Senate, the Senate must take them up at 1 p.m. the next day (unless it is a Sunday) and consider them six days a week (except Sundays) until it renders a judgment.
The chief justice of the Supreme Court presides over the trial, but the format is left to the Senate itself, which can proceed in several ways: The Republican and Democratic leaders may strike an agreement on one or more resolutions governing the proceedings, or, failing that, the resolutions may be adopted by a simple majority of 51 senators. Or there can be no resolutions at all, and senators can introduce motions as they go.
With the Senate split 53 to 47 between Republicans and Democrats (including the two independents who vote with Democrats), Mr. McConnell has a thin margin. If Democrats stick together and four Republicans defect, Mr. McConnell could lose control of the proceedings. Viewed another way, he can lose only two votes if he wants to push through a resolution with only Republican support.
Because the trial is unfolding 10 months before the 2020 presidential election, that makes for tricky politics. Mr. McConnell must be mindful of the wishes of vulnerable Republicans like Senators Susan Collins of Maine and Cory Gardner of Colorado, whose seats he wants to protect. If the Senate appears to be giving the impeachment short shrift, or the trial turns into a spectacle instead of a solemn constitutional proceeding, those Republicans will suffer.
But Republicans are not the only ones concerned about politics. Mr. Schumer, too, must worry about vulnerable members of his rank-and-file, notably Senator Doug Jones, who is seeking re-election in Alabama.
In an interview on Fox News on Thursday evening, Mr. McConnell said that there was “no chance” that the president would be removed from office and that he would follow Mr. Cipollone’s lead.
“Everything I do during this, I’m coordinating with the White House counsel,” Mr. McConnell said. “There will be no difference between the president’s position and our position as to how to handle this to the extent that we can.”
In a sense, the Trump impeachment is a repeat of the impeachment of President Bill Clinton in 1998. Mr. Clinton, a Democrat who was accused of perjury and lying under oath to cover up his affair with an intern, Monica Lewinsky, was impeached by Republicans and acquitted by Democrats. Mr. Trump, a Republican, is on track to being impeached by Democrats and acquitted by Republicans.
But even before it begins, Mr. Trump’s Senate trial appears to be a more partisan affair. Mr. McConnell has yet to consult with Mr. Schumer on the parameters of the trial, though he has said he would do so.
By contrast, Tom Daschle, the Democratic majority leader of the Senate when Mr. Clinton was impeached, and Trent Lott, the Republican leader, issued a joint statement after the House approved impeachment articles, promising a fair and impartial trial. They worked closely together throughout, Mr. Lott said in an interview on Friday.
“One thing I knew from the get-go was we were not going to remove Bill Clinton,” he said, “so the challenge is to have a process that everybody felt was fair.”
Mr. Lott said he studiously avoided commenting on the articles of impeachment, and pushed back on pressure from his rank-and-file to have Ms. Lewinsky and other witnesses testify in the well of the Senate; he believed it would “demean” the proceedings. Instead, they gave videotaped depositions.
“I wanted to set it up with the proper decorum and seriousness and, as much as possible bipartisanship,” Mr. Lott said, “so that when we did get to the end, the Republicans who hated Bill Clinton would say, ‘Well, we didn’t remove him, but we did our job.’ ”
Nicholas Fandos contributed reporting from Washington, and Maggie Haberman from New York.