President Donald Trump has lashed out over his impending impeachment in an irate letter to top Democrat Nancy Pelosi, accusing her of declaring “open war on American democracy”.

“You have cheapened the importance of the very ugly word, impeachment!” he wrote in the letter, sent on Tuesday.

Mr Trump faces an impeachment vote on Wednesday over allegations he pressured Ukraine for personal political gain.

He is expected to be impeached, setting up a trial in the Senate.

With little hope of changing the outcome of Wednesday’s vote in the House, Mr Trump used his six-page letter to angrily rail against the process and denounce Ms Pelosi, the Democratic Speaker of the House.

It was a remarkable intervention by the president, who has fought to stymie the impeachment process by preventing key aides from testifying before the House of Representatives.

What else did Trump’s letter say?

Mr Trump claimed in his letter he had been “deprived of basic Constitutional Due Process from the beginning of this impeachment scam” and “denied the most fundamental rights afforded by the Constitution, including the right to present evidence”.

Ms Pelosi told reporters at the Capitol that she hadn’t read the letter in full but had seen “the essence” of it and thought it was “really sick”.

In a statement announcing Wednesday’s vote on impeachment, she said the House would “exercise one of the most solemn powers granted to us by the constitution”.

“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,” she added.

Demonstrators carried placards bearing the words “Dump Trump”, and “Protect our Democracy”.

The hashtags #notabovethelaw and #impeachmenteve trended on Twitter.

What happens on Wednesday?

Mr Trump is facing two impeachment charges: obstruction of Congress, by refusing to co-operate with the impeachment probe, barring staff from testifying, and holding back documentary evidence; and attempting to use his office to pressure Ukraine to investigate his Democratic political rival Joe Biden.

If the House votes as expected on Wednesday along party lines, Mr Trump will become the third president in US history to be impeached. He will then go on trial in the Senate, where Senators from both parties are obliged to act as independent jurors.

Mr Giuliani told the New York Times he passed along to Mr Trump “a couple of times” information about how Ms Yovanovitch had got in the way of potential investigations.

“I needed Yovanovitch out of the way,” Mr Giuliani told the New Yorker magazine.