Trump warns if Democrats impeach, they'll be 'declaring war on American democracy'

On the eve of his expected impeachment in the U.S. House of Representatives, 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 fiery letter he signed addressed to House Speaker Nancy Pelosi, and released as House lawmakers met to set the rules for debate ahead of Wednesday's planned vote on two articles of impeachment — formal charges — against Trump.

"This is nothing more than an illegal, partisan attempted coup that will, based on recent sentiment, badly fail at the voting booth," Trump's letter stated, alluding to the 2020 U.S. presidential election in which he is seeking another four years in office.

The U.S. 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 the Republican president 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 added.

House Democrats accuse Trump of abusing his power by asking Ukraine to investigate Biden, a former U.S. 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.

"Look, this has been a total sham from the beginning," Trump told reporters at the White House shortly after his letter was released.

Meanwhile, members of the House rules committee met on Tuesday over the rules for the debate before the vote set for Wednesday by the full House, which is expected to result in Trump becoming the third U.S. president to be impeached.

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

Earlier, the Senate's top Republican, Mitch 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 duelling 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 favoured a coverup.

While McConnell said on the floor it was the Senate's role simply to act as a "judge and jury," he later told reporters that he is not "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's aide Robert Blair and budget official Michael Duffey. Schumer has argued that such testimony could sway Republicans in favour of removing Trump.

Trump has refused to co-operate 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 very soon to discuss how to proceed. The White House also indicated opposition to Schumer's requests for the Senate trial.

"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.