US President Donald Trump has penned a furious letter to House Speaker Nancy Pelosi, denouncing a “vicious crusade” against him.

The letter came a day before the historic House vote on Mr Trump’s impeachment, and as Democrats tallied votes in favour of the move and Republicans looked ahead, vowing to defend the President at Senate trial.

Mr Trump, who would be just the third US president to be impeached, acknowledged he was powerless to stop the Wednesday’s (local time) vote.

The House is almost certain to vote in favour of the impeachment. That means proceedings will move to a trial in the Republican-dominated Senate in January – at which the President is widely expected to be acquitted.

Mr Trump appeared to intend his six-page accusatory message less for Ms Pelosi than the broad audience of citizens – including 2020 voters – watching history unfolding on Capitol Hill.

“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,” Mr Trump wrote.

“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.”

NEW: @realDonaldTrump send angry letter to @SpeakerPelosi protesting impeachment. pic.twitter.com/qsLfZjAOdp

— Andrew Feinberg (@AndrewFeinberg) December 17, 2019

He accused Democrats of acting out of “Trump Derangement Syndrome”, and still smarting from the 2016 election loss.

“You are the ones bringing pain and suffering to our Republic for your own selfish, personal political and partisan gain,” he wrote.

Portraying himself as blameless, Mr Trump compared the inquiry to the Salem Witch Trials, accusing Democrats of a “perversion of justice and abuse of power”.

At an Oval Office media conference shortly after the letter became public, Mr Trump was asked if he bore any responsibility for the impeachment proceedings.

“No, I don’t think any. Zero, to put it mildly,” he said.

Ms Pelosi, who warned earlier this year against pursuing a strictly partisan impeachment, nonetheless has the numbers in the House.

“Very sadly, the facts have made clear the President abused his power for his own personal, political benefit and he obstructed Congress,” she wrote to colleagues. “In America, no one is above the law.

“During this very prayerful moment in our nation’s history, we must honour our oath to support and defend our constitution from all enemies, foreign and domestic.”

No Republicans have indicated they will support the two articles of impeachment, for abuse of power and obstruction of Congress, setting up a close-to-party-line vote.

Senate Majority Leader Mitch McConnell set the partisan tone for the next step, as attention will shift to the Senate.

“I’m not an impartial juror,” he declared.

Democrat “leadership,” despite their denials, are putting tremendous pressure on their members to vote yes on this ridiculous Impeachment. If they vote yes, it will be much easier for Republicans to win in 2020!

— Donald J. Trump (@realDonaldTrump) December 17, 2019

Mr Trump is accused of asking the new president of Ukraine, an ally facing an aggressive Russia at its border, in July to “do us a favour” by investigating Democrats, including potential 2020 rival Joe Biden.

At the time, President Volodymyr Zelenskiy was hoping for a coveted White House meeting and nearly $US400 million ($A584 million) in military aid Congress had approved to counter Russia.

The White House put the money on hold as leverage, the Democrats say.

In his letter on Tuesday, Mr Trump defended his “absolutely perfect” phone call that sparked the impeachment inquiry.

“You are the ones interfering in America’s election. You are the ones subverting America’s Democracy. You are the ones Obstructing Justice,” he wrote.

“You are the ones bringing pain and suffering to our Republic for your own selfish personal, political, and partisan gain.”

“I have no doubt the American people will hold you and the Democrats fully responsible in the upcoming 2020 election. They will not soon forgive your perversion of justice and abuse of power.”

He also tried to justify anew the Ukrainian investigations he wanted into Mr Biden. And he disputed the charges of abuse of power and obstruction of Congress’ investigation.

-with AAP