WASHINGTON — An enraged US President Donald Trump said he was being subjected to an "attempted coup" and a witch trial as Democrats set a historic impeachment vote for Wednesday.

In an extraordinarily angry six-page letter, Trump on Tuesday told Nancy Pelosi, speaker of the Democratic-led House of Representatives, that "history will judge you harshly".

Referring to a famous miscarriage of justice and religious extremism in the 17th-century United States, resulting in 20 executions, Trump said he'd been given less rights than "those accused in the Salem Witch Trials".

The letter came just minutes before Pelosi announced that the House would vote on Wednesday to make Trump only the third US leader ever impeached and placed on trial in the Senate.

"Tomorrow the House of Representatives will exercise one of the most solemn powers granted to us by the Constitution as we vote to approve two articles of impeachment against the president of the United States," House Speaker Nancy Pelosi said in a letter to Democratic colleagues on Tuesday.

"During this very prayerful moment in our nation's history, we must honor our oath to support and defend our constitution from all enemies, foreign and domestic," she added.

Frustration and fury

Trump is accused of attempting to force Ukraine into what would have been a damaging announcement of an unfounded probe into a main 2020 reelection rival, Joe Biden.

He is also accused of obstructing Congress by refusing to cooperate with the impeachment investigation, barring staff from testifying and holding back documentary evidence.

The two articles of impeachment are certain to pass in the House, where Democrats hold a firm majority.

That will send the case to the Senate, where a trial of Trump is expected to open in January, and his acquittal is equally expected, given the Republicans' control there.

Even with that likely outcome, Trump virtually exploded in an extraordinary outpouring of frustration and fury in the letter to Pelosi defending his record and attacking Democrats.

The letter, on White House paper and ending with his characteristic oversized signature in thick black pen, accused the veteran Democratic politician of "breaking your allegiance to the Constitution" and "declaring open war on American Democracy."

It repeated his claim that the entire case against him is a "hoax" and a "colossal injustice."

It said Democrats were being driven in impeachment "by your most deranged and radical representatives of the far left." — AFP