The speaker’s statement effectively ended an impasse over the impeachment process that had left the president’s Senate trial in limbo for weeks. She did not announce which Democrats would manage the case, but said the House should be ready to appoint them next week and to formally deliver to the Senate charges of abuse of power and obstruction of Congress.

“I have asked Judiciary Committee Chairman Jerry Nadler to be prepared to bring to the floor next week a resolution to appoint managers and transmit articles of impeachment to the Senate,” Pelosi said in a letter to colleagues. “I will be consulting with you at our Tuesday House Democratic Caucus meeting on how we proceed further.”

Throughout the delay, the speaker had insisted that she was merely pushing for a fairer Senate proceeding after Senator Mitch McConnell, Republican of Kentucky and the majority leader, promised publicly to collaborate with Trump’s legal team to secure a quick acquittal. Democrats claimed the maneuver successfully spotlighted the need for the Senate to hear from witnesses and see documents that Trump barred from the House impeachment inquiry.

But she ultimately failed to win any concessions from Republicans on the terms of the trial, and some Democrats, including the senior senator from her home state, Dianne Feinstein, wondered aloud in recent days about the point of delaying the trial, before taking back her remarks.

As recently as Thursday, Pelosi had refused to say when she would act and demanded one final time that . McConnell share the precise rules for a Senate trial so she could tailor her prosecutorial team. But . McConnell never appeared to even consider committing to that approach, and he said this week that he had secured the votes he needed to begin a trial on his own terms, without an agreement on witnesses or documents.

Under the timetable the speaker suggested on Friday, the Senate’s proceeding would begin promptly — as soon as Wednesday.

. Trump’s acquittal appears all but certain in the Republican-led chamber. But the trial could plunge Congress, the presidency and the 2020 presidential campaign into uncertainty for weeks. Democrats have made clear they intend to force votes on whether to call witnesses, and are pressuring Senate Republicans — particularly moderates and those who face re-election challenges in politically competitive states — to join them in supporting the airing of more information.

“In an impeachment trial, every senator takes an oath to ‘do impartial justice according to the Constitution and laws,’” Pelosi said in her letter. “Every senator now faces a choice: to be loyal to the president or the Constitution.”

The proceeding could also keep close to half of the Democratic presidential contenders in the Senate chamber during the critical days leading to the Iowa caucus on Feb. 3, the first contest of the primary cycle.

Though officials at both ends of Pennsylvania Avenue have been preparing for weeks, Pelosi’s letter on Friday began an unofficial countdown toward opening arguments. . Trump and his legal team were still sorting out who would mount his defense in the Senate chamber. And the speaker continued to assemble her own team of managers to prosecute him.

The Democratic-led House impeached . Trump on Dec. 18, charging him with abuse of power and obstruction of Congress in connection with a scheme to pressure Ukraine to publicly investigate his domestic political rivals. Specifically, after months of investigation and public testimony, the House concluded that . Trump withheld about $400 million in military assistance and a coveted White House meeting for Ukraine as leverage to extract investigations that could bolster his re-election campaign, and then sought to conceal it from Congress with an unprecedented campaign of obstruction.

On the night of the vote, Pelosi unexpectedly announced she would not immediately send the articles of impeachment to the Senate in an attempt to pressure the Republican-led chamber to commit to calling additional witnesses and requesting documents that . Trump blocked during the House’s inquiry. A trial with no new evidence, Democrats have argued, would fundamentally abet what they describe as the president’s cover-up.

In her letter, Pelosi charged that . McConnell “has been engaged in tactics of delay in presenting transparency, disregard for the American people’s interest for a fair trial and dismissal of the facts.”

As weeks passed, Pelosi’s move prompted debates about how much leeway the Constitution allows each chamber in the impeachment process.

For his part, . McConnell has repeatedly condemned the House’s case as rushed and woefully inadequate, without addressing the behavior of which . Trump is accused.

His response to the speaker’s news on Friday was terse. “About time,” he told reporters.

. McConnell has yet to make public his proposed rules for the trial, but he said they would be modeled on a resolution guiding the 1999 trial of President Bill Clinton.

To Democrats’ dismay, that model puts off any decisions on calling witnesses or new evidence until the middle of the trial, after senators are sworn in, the House and White House present opening arguments and senators have a chance to ask written questions. Nor does it guarantee that new evidence will be included.

The minority leader, Senator Chuck Schumer of New York, said Friday that Democrats “would do everything we can to see that the truth comes out.”

Democrats are closely watching a small group of moderate Republicans who are open to calling witnesses, hoping to court their support. With the chamber divided 53 to 47, they need four Republicans to cross party lines if they want a shot at hearing from officials like John R. Bolton, . Trump’s former national security adviser, who are said to have pertinent information about the president’s actions toward Ukraine but evaded House investigators.

Complicating matters, . Trump told Fox News on Friday that he would probably invoke executive privilege to try to shield . Bolton’s testimony if the Senate summoned him. . Trump said he had no problem with what . Bolton might say, but that “for the sake of the office” of the president, he did not want to set a standard of letting a top adviser speak about their interactions.

. Bolton indicated in a statement this week that he would testify if subpoenaed, setting up a potential legal clash.

Even if the trial were to begin Wednesday, it could take several days to be fully organized. Officials in both chambers suggested on Friday that the heat of the trial — beginning with up to 24 hours or oral arguments per side — could begin shortly after the Martin Luther King’s Birthday on Jan. 20. If a majority of senators do vote to call witnesses, that could extend the proceeding by several weeks.

. Trump’s team is likely to be led by Pat Cipollone, the White House counsel, who vigorously enforced the president’s stonewall of the House inquiry and refused to mount a defense in that chamber despite invitations to do so. He will probably be assisted by two White House deputies and Jay Sekulow, one of . Trump’s personal lawyers, according to two people familiar with the matter who were not authorized to speak publicly.

Some people close to . Trump have also pushed to give public roles to three House Republicans — Doug Collins of Georgia, John Ratcliffe of Texas and Jim Jordan of Ohio — television-savvy conservatives who led the defense in the House. But . McConnell and Senator Lindsey Graham, Republican of South Carolina, have counseled that senators could find it off-putting.

Pelosi’s team will almost certainly be led by . Nadler and Representative Adam B. Schiff of California, the Intelligence Committee chairman, who together oversaw the House’s impeachment inquiry in the fall. But the speaker is still trying to piece together a regionally and racially diverse team that will appeal to senators.

Despite winning no commitments from . McConnell, Democrats argued that Pelosi’s delay strategy had payoffs.

During the intervening three weeks between the House vote and Pelosi’s announcement, relevant new documents that . Trump suppressed have come to light, suggesting that there is additional evidence to support the charges the House brought.

Still, after months of controlling the narrative in the news media, Pelosi exposed herself to second-guessing. Republicans accused her of hypocrisy for waiting to prosecute . Trump after months of insisting that he posed an urgent threat to the integrity of the 2020 election that must be addressed with a speedy impeachment vote. And some Democrats privately worried that argument could gain traction with the general public, undermining hard work in the House.