US 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”, reports BBC.

“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.

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”.

“More due process was afforded to those accused in the Salem Witch Trials,” he wrote.

The president was in fact publicly invited by the Democratic chair of the House Judiciary Committee to give evidence in the impeachment process, which would have also allowed his legal team to question witnesses, but he declined.

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.