An impeachment trial against Donald Trump will be set in motion in the Senate next week, according to a letter sent on Friday by the House speaker, Nancy Pelosi, to colleagues.

A two-thirds majority of senators voting to convict Trump on either of two articles of impeachment would result in his removal from office, although that is considered a long shot given the Republicans’ firm control of the Senate.

“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 wrote in the letter.

Impeachment “managers” will prosecute the case against Trump in the Senate. Last month the House approved two articles of impeachment against Trump, one for abuse of power and one for obstruction of Congress.

While Trump’s survival seemed assured, new signs of potential hazards in the process for Trump emerged on Friday, with the revelation by the Maine senator Susan Collins, a moderate Republican locked in a tough re-election fight, that she had been working all week with a “fairly small group” of fellow Republican senators to allow for witnesses at the trial.

“We should be completely open to calling witnesses,” Collins was quoted as saying by Bangor Daily News, which first published the report.

The Senate majority leader, Mitch McConnell, and other leading Republicans have stated categorically that no witnesses should be called in the trial, framing the impeachment as a “sham” and defending Trump’s conduct as irreproachable.

But just four Republicans would be needed to create a simple majority, with the 47-member Democratic caucus, to force the appearance of witnesses and the inclusion of documents and other evidence in the trial.

After the articles are transmitted to the Senate, McConnell has signaled that he wishes to open and close a trial quickly, moving this week to settle on rules for the trial without any input from Democrats.

McConnell had accused Pelosi of delaying the transmission of the articles out of a sense of embarrassment on the part of Democrats, he said, in the weakness of the case.

But in her letter, Pelosi detailed new evidence to emerge since impeachment of a plot by Trump to extort Ukraine into making negative headlines about Joe Biden, his political rival. The evidence included internal White House and Pentagon emails documenting the circumstances of the holdup in nearly $400m in aid to Ukraine.

Pelosi had said she was waiting to transmit the articles to the Senate until McConnell made clear the terms of a trial. Republicans including McConnell have pre-emptively opposed hearing from witnesses and reviewing evidence at the trial, and said they would not act as “impartial jurors” despite an oath of impartiality they will be required under Senate rules to swear.

“Yesterday, he [McConnell] showed his true colors and made his intentions to stonewall a fair trial even clearer by signing on to a resolution that would dismiss the charges. A dismissal is a cover-up and deprives the American people of the truth,” Pelosi wrote.

Pressure had been growing on Pelosi, including from other Democrats, to transmit the articles in the third week after impeachment. The House returned from a holiday break at the start of the week.

The announcement appeared to set up a Senate trial to begin sometime after a Democratic debate scheduled for Tuesday night, which would include the participation of four senators running for president: Elizabeth Warren, Bernie Sanders, Amy Klobuchar and Cory Booker.

Once the Senate trial begins, senators are required to be present in Washington for proceedings scheduled to run every day except Sunday until the trial is finished.

“I will be consulting with you at our Tuesday House Democratic Caucus meeting on how we proceed further,” Pelosi wrote to her colleagues.

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