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

"It is time for you and the highly partisan Democrats in Congress to immediately cease this impeachment fantasy and get back to work for the American People," Trump wrote in a six-page letter, adding that he has "no expectation" that House Democrats will do so.

Trump's remarks came in a signed letter addressed to House Speaker Nancy Pelosi, released as House lawmakers met to set the rules for debate ahead of Wednesday's planned vote on two articles of impeachment, accusing Trump of abusing power and obstruction of Congress.

If any of the two articles are approved, Trump will be the third president in the U.S. history to be formally impeached and face trial in the Senate.

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Trump accused Pelosi of "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, according to the president, is an "an illegal, partisan attempted coup that will, based on recent sentiment, badly fail at the voting booth," alluding to the November 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, while the Senate shall have the sole power to try all impeachments.

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.

'You are declaring war on democracy!'

In the letter, Trump defended his conduct toward Ukraine, accusing Democrats of "interfering in America's elections."

"You are the ones interfering in America's elections. You are the ones subverting America's Democracy. You are the ones Obstructing Justice," Trump wrote, "You are the ones bringing pain and suffering to our Republic for your own selfish personal, political, and partisan gain."

Trump described the impeachment as "invalid," rebuking Pelosi's move is "declaring open war on American Democracy," Trump wrote. "You view democracy as your enemy!"

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.

Impeachment is only a political decision

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, signaling 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.

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."

(With input from Reuters, Xinhua)