The Democratic-controlled House of Representatives was expected to pass two articles of impeachment against US President Donald Trump on Wednesday, charging him with abuse of power and obstruction of Congress, which would mark only the third time in US history that a president has been impeached.

Thousands of protesters rallied in front of the San Francisco Federal Building in support of the impeachment on Tuesday night, the eve of the House vote. Organizers said more than 2,200 people turned out for the event. Similar protests also took place in other Bay Area cities such as Oakland, San Jose, Palo Alto, Walnut Creek and Redwood City.

In a six-page letter to US House Speaker Nancy Pelosi, Trump urged her to halt impeachment proceedings, accusing Democrats of an "unprecedented and unconstitutional abuse of power".

"While I have no expectation that you will do so, I write this letter to you for the purpose of history and to put my thoughts on a permanent and indelible record," Trump wrote.

He accused Pelosi of having "cheapened the importance of the very ugly word, impeachment" and said she was "declaring open war on American Democracy" by pursuing his impeachment.

"You dare to invoke the Founding Fathers in pursuit of this election-nullification scheme－yet your spiteful actions display unfettered contempt for America's founding and your egregious conduct threatens to destroy that which our Founders pledged their very lives to build," Trump wrote.

After the House votes on the two charges on Wednesday, it would set the stage for a trial in the Senate, which is expected to begin in January. Republican and Democratic senators clashed on Tuesday over how to conduct the Senate trial that decides whether Trump remains in power.

Senate Majority Leader Mitch McConnell rejected a Democratic request to call four current and former White House officials as witnesses in the Senate trial.

Senate Democratic leader Chuck Schumer said he wants the trial to consider documents and hear testimony from former national security adviser John Bolton, acting White House Chief of Staff Mick Mulvaney, Mulvaney aide Robert Blair and budget official Michael Duffey. Schumer said a trial without witnesses would be a "sham" and suggested Trump's fellow Republicans favored a cover-up.

But McConnell said he would not allow a "fishing expedition" after what he called a "slapdash" House impeachment process.

"So now, the Senate Democratic leader would apparently like our chamber to do House Democrats' homework for them. And he wants to volunteer the Senate's time and energy on a fishing expedition to see whether his own ideas could make Chairman Schiff's sloppy work more persuasive than Chairman Schiff himself bothered to make it," McConnell said.

Adam Schiff is the chairman of the House Intelligence Committee that led the impeachment inquiry launched in September.

"From everything we can tell, House Democrats' slapdash impeachment inquiry has failed to come anywhere near－anywhere near－the bar for impeaching a duly elected president, let alone removing him for the first time in American history," McConnell said.

Trump has refused to cooperate with the House impeachment process and ordered current and former officials like those mentioned by Schumer not to testify or provide documents.

Moments after McConnell made his comments, Schumer took to the Senate floor. "What is Leader McConnell afraid of? What is President Trump afraid of? The truth?" he asked.

"If you're trying to conceal evidence and block testimony, it's probably not because the evidence is going to help your case. It's because you're trying to cover something up."

Schumer vowed to demand votes by senators on whether to call witnesses and subpoena documents during the trial.

The AP, Reuters and Xinhua contributed to this story.