It was only a few weeks ago that the top Senate Republican was hinting that his chamber would make short work of impeachment.

But this week, Senator Mitch McConnell sat his colleagues down over lunch in the Capitol and warned them to prepare for an extended impeachment trial of President Trump.

According to people who were there, he came equipped with a PowerPoint presentation, complete with quotes from the Constitution, as he schooled fellow senators on the intricacies of a process he portrayed as all but inevitable.

Few Republicans are inclined to convict Mr. Trump on charges that he abused his power to enlist Ukraine in an effort to smear his political rivals. Instead, Mr. McConnell sees the proceedings as necessary to protect a half a dozen moderates in states like Maine, Colorado and North Carolina who face re-election next year and must show voters they are giving the House impeachment charges a serious review.

It’s people like Senator Susan Collins of Maine who will be under immense political pressure as they decide the president’s fate.

“To overturn an election, to decide whether or not to convict a president is about as serious as it gets,” Ms. Collins said.

Mr. McConnell is walking a careful line of his own in managing the fast-moving impeachment process. On Friday, the senator wrote a scathing op-ed criticising the president’s decision to pull back troops from northern Syria, calling it a “grave strategic mistake.” But Mr. McConnell views it as his role to protect a president of his own party from impeachment and in a recent fund-raising video, he vowed to stop it.

The mood among Republicans on Capitol Hill has shifted from indignant to anxious as a parade of administration witnesses has submitted to closed-door questioning by impeachment investigators and corroborated central elements of the whistle-blower complaint that sparked the inquiry.

They grew more worried still on Thursday, after Mick Mulvaney, the acting White House chief of staff, undercut the president’s defense by saying that Mr. Trump had indeed withheld security aid from Ukraine in order to spur an investigation of his political rivals. Mr. Mulvaney later backtracked, but the damage was done.

“I couldn’t believe it – I was very surprised that he said that,” said Representative Francis Rooney, Republican of Florida, who mocked Mr. Mulvaney’s attempts to take back comments that had been broadcast live from the White House briefing room.

“It’s not an Etch A Sketch,” Mr. Rooney said, miming the tipping movement that erases the toy drawing board. “There were a lot of Republicans looking at that headline yesterday when it came up, I certainly was.”

Senator Lisa Murkowski – an Alaskan Republican who is seen as potentially open to removing Mr. Trump from office – told reporters that a president should never engage in the kinds of actions that Mr. Mulvaney appeared to acknowledge.

“You don’t hold up foreign aid that we had previously appropriated for a political initiative,” she said. “Period.”

Still, Republicans said they did not detect a significant shift that would pose a serious threat to the president in the Senate. It would require 20 Republicans to side with Democrats in convicting Mr. Trump, and few observers believe that will happen.

Mr. McConnell, his allies said, regards the impeachment fight in much the same way as he did the struggle last year to confirm Judge Brett M. Kavanaugh to the Supreme Court, in which he was primarily concerned with protecting his Senate majority by insulating vulnerable incumbents. Then, as now, they said, Mr. McConnell is focused on keeping Republicans as united as possible, while allowing those with reservations about Mr. Trump’s conduct and their own political considerations to justify their decision to their constituents.

“I think he will play it straight,” said Senator John Cornyn, Republican of Texas and a close McConnell ally, who noted his party’s narrow voting margin. “I don’t think he has any alternative. When you are operating with 53 you have thin margins and you can’t jam anybody or you end up with undesirable consequences.”

Mr. McConnell has told colleagues he expects the House to impeach Mr. Trump quickly, possibly by Thanksgiving, an educated hunch based on the pace of the inquiry so far and Speaker Nancy Pelosi’s decision to keep the inquiry narrowly focused on Mr. Trump’s dealings with Ukraine. He plans to move swiftly too, he told colleagues, using the approach of Christmas to force the Senate to complete its work before the beginning of 2020.

Yet an impeachment trial is a spectacle that is by its nature unpredictable, and most of the senators who will act as jurors were not around for the last one, of Bill Clinton in 1999. Mr. McConnell reminded senators that Chief Justice John G. Roberts Jr. would preside over the trial, and would have wide latitude in handling motions that might be made, including any motion to dismiss the charges that Republicans might try to put forward to short circuit the process.

Mr. McConnell’s declaration that the Senate would move forward was in part designed to show he had no choice, an effort to deflect criticism from conservatives outraged that the Senate would even consider impeachment.

On Wednesday, Senator Lindsey Graham, Republican of South Carolina, pushed for Senate Republicans to write a letter to Ms. Pelosi declaring that they would not remove the president. But some senators raised objections, worrying that some of their colleagues would not want to sign on, a result that would expose disunity among Republicans. Mr. Graham’s colleagues said they believe they staved off the letter, which they viewed as a mistake.

Mr. McConnell has made it clear that he plans to sit down with Senator Chuck Schumer of New York, the Democratic leader, to see if they can find a mutually acceptable way to move forward as Democrats and Republicans did in 1999 when they unanimously agreed on the framework for the impeachment trial. The Senate is much more polarised now, though Mr. Schumer this week held out hope.

“We have to do this trial in a fair and bipartisan way and I hope that Leader McConnell would obey those strictures,” Mr. Schumer said. In the battle for Senate control, Democrats have their own political risks to consider since impeachment could prompt a backlash against some of their candidates if enough voters conclude that the president was pursued unfairly.

Just 15 senators remain in office from the time Mr. Clinton was put on trial. Mr. McConnell warned them of the weight of the trial, where they can be required to be on the floor all afternoon six days a week without speaking — a major challenge for senators who relish their chances to be heard.

“It will mean day after day sitting in chamber, listening to the two sides, writing questions for them to answer that go through the chief justice,” said Ms. Collins, one of the Republicans who voted to acquit Mr. Clinton 20 years ago. “Members who have not been through this before will find it is a great deal of work.”

•Text (excluding amended headline: New York Times.

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