Washington: The US House of Representatives began debate on Wednesday ahead of a historic vote on two articles of impeachment against President Donald Trump, who is poised to become the nation's third president to be impeached.

The vote on charges against the Republican president, expected to come later on Wednesday, is expected to fall along party lines in the Democratic-controlled chamber.

Meanwhile, President Donald Trump accused Democrats of pursuing an "illegal, partisan attempted coup" and declaring war on American democracy as they seek to remove him from office for pressing Ukraine to investigate political rival Joe Biden.

Trump's remarks came in a signed letter addressed to House Speaker Nancy Pelosi, released as House lawmakers set the rules for debate ahead of Wednesday's planned vote on two articles of impeachment - formal charges - against the Republican president.

At the same time, the Senate's top Republican, Mitch McConnell, signalled he would not allow the expected upcoming Senate trial on impeachment charges to involve more fact-finding about Trump's conduct. The Republican-led Senate is unlikely to vote to convict the president.

The rambling, six-page letter from Trump to Pelosi on White House letterhead largely restated the president's objections to the impeachment probe, but did so in accusatory and sometimes spiteful language that attacked Pelosi, congressional Democrats, Biden and institutions such as the FBI.

Pelosi, Trump wrote, "is turning the House of Representatives from a revered legislative body into a Star Chamber of partisan persecution" while "scarcely concealing your hatred of me."

The impeachment probe, the president said, was an "an illegal, partisan attempted coup that will, based on recent sentiment, badly fail at the voting booth," alluding to the November 2020 US presidential election in which he is seeking another four years in office.

The US Constitution gives the House the power to impeach a president for "high crimes and misdemeanors," part of the document's checks and balances among the executive, legislative and judicial branches of the federal government.

The Democratic-led House is expected to pass two articles of impeachment charging Trump with abuse of power and obstruction of Congress for his dealings with Ukraine

"By proceeding with your invalid impeachment, you are violating your oaths of office, you are breaking your allegiance to the Constitution, and you are declaring open war on American Democracy," Trump wrote.

"You view democracy as your enemy!" he wrote.

Asked about Trump's letter, Pelosi told CNN: "I haven't fully read it. We've been working. I've seen the essence of it though. It's really sick."

Senate clash over witnesses

On the eve of the vote, Pelosi sent a letter to all 232 Democratic members of the House, urging them to defend the Constitution.

"Very sadly, the facts have made clear that the President abused his power for his own personal, political benefit and that he obstructed Congress as he demanded that he is above accountability, above the Constitution and above the American people," she said.

House Democrats accuse Trump of abusing his power by asking Ukraine to investigate Biden, a former US vice president and a leading Democratic contender to oppose him in the 2020 election. Trump is also accused of obstructing the congressional investigation into the matter.

Members of the House Rules Committee set a six-hour limit for debate before the vote on Wednesday by the full House, which is expected to result in Trump becoming the third US president to be impeached.

No president has ever been removed from office via the impeachment process set out in the Constitution.

Earlier, McConnell brushed aside a Democratic request to call four current or former White House officials as witnesses in the Senate impeachment trial expected next month, again making clear that he expects senators not to remove Trump from office.

In dueling speeches on the Senate floor, McConnell said he would not allow a "fishing expedition" after a "slapdash" House impeachment process, while Senate Democratic leader Chuck Schumer said a trial without witnesses would be a "sham" and suggested Trump's fellow Republicans favored a cover-up.

McConnell said on the floor it was the Senate's role simply to act as a "judge and jury," but he later told reporters that he would not be "an impartial juror."

"This is a political process. There is not anything judicial about it," he said. "Impeachment is a political decision."

Schumer said he was "utterly amazed" by McConnell's remark. He had said he wants the trial to consider documents and hear testimony from four witnesses: former national security adviser John Bolton, acting White House Chief of Staff Mick Mulvaney, Mulvaney aide Robert Blair and budget official Michael Duffey.

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.

McConnell and Schumer both said they expected to meet soon to discuss how to proceed.

"What is Leader McConnell afraid of? What is President Trump afraid of? The truth?" Schumer asked on the Senate floor.

"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 added.