US president Donald Trump is on the cusp of being impeached by the house of representatives.

A historic debate is due to begin at 9am (2pm GMT) on charges that he abused his power and obstructed US congress.

Votes will then be held, which will leave a defining mark on Mr Trump’s tenure at the White House.

Mr Trump, who would be just the third US president to be impeached, fired off a furious letter to house speaker Nancy Pelosi on Tuesday, denouncing the “vicious crusade” against him.

He also acknowledged he was powerless to stop the expected outcome.

The president wrote: “When people look back at this affair, I want them to understand it, and learn for it, so that it can never happen to another president again.”

Early on Wednesday, Mr Trump tweeted his outrage: “Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG! A terrible thing.”

Ms Pelosi, who warned earlier this year against pursuing a strictly partisan impeachment, nonetheless has the necessary numbers from Democrats to approve it.

According to a tally compiled by The Associated Press, Mr Trump is on track to be formally charged by a house majority.

Nancy PelosiSpeaker Nancy Pelosi with fellow Democrats (AP)

Ms Pelosi wrote to colleagues: “Very sadly, the facts have made clear that the president abused his power for his own personal, political benefit and that he obstructed congress.

“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.”

The rare undertaking to impeach a president, set to unfold over more than six hours of debate on Wednesday, is splitting US congress much the way Americans have different views of Mr Trump’s unusual presidency and the articles of impeachment against him.

From Alaska to Florida, tens of thousands of Americans marched in support of impeachment, from a demonstration through a rainy Times Square to handfuls of activists standing vigil in small towns.

They carried signs saying “Save the Constitution – Impeach!!!!” and “Criminal-in-Chief.”

Mr Trump implores Americans to “read the transcript”, but the facts of his phone call with the Ukraine president are not necessarily in dispute.

The American leader asked Volodymyr Zelenskiy to investigate Democrats and his 2020 political rival Joe Biden.

At the time, the newly elected Ukrainian leader was hoping for a coveted White House visit to showcase his standing with the US, his country’s most important ally.

Trump Impeachment IowaDemonstrators with a sign reading “We believe in the Constitution” during an “Impeach and Remove” rally in Iowa (AP)

He was also counting on nearly 400 million dollars (£303 million) in military aid as his country confronts a hostile neighbour, Russia.

The question for members of congress, and Americans, is whether those actions, and the White House’s block on officials testifying for the house investigation, are impeachable offences.

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

On the eve of the House debate, Mr Trump appeared to target his lengthy, accusatory message less towards Ms Pelosi than for the broad audience of citizens — including 2020 election voters — watching history unfolding on Capitol Hill.

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

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

Trump Impeachment Protest-NYCAn anti-Trump rally in New York (AP)

Portraying himself as a blameless victim, Mr Trump compared the impeachment inquiry to the “Salem Witch Trials”.

Asked later if he bore any responsibility for the proceedings, he said: “No, I don’t think any. Zero, to put it mildly.”

But the house impeachment resolution says Mr Trump abused the power of his office and then tried to obstruct the investigation in congress like “no other” president in history.

Mr Trump “betrayed the Nation by abusing his high office to enlist a foreign power in corrupting democratic elections”, the resolution says.

“President Trump, by such conduct, has demonstrated that he will remain a threat to national security and the constitution if allowed to remain in office.”

Ahead of House votes, one by one, centrist Democratic members of congress, including many first-term freshmen who built the House majority and could risk their reelection in districts where the president is popular, announced they would vote to impeach.

Republicans disagreed, firmly.

Senate majority leader Mitch McConnell set the partisan tone for the next step, as attention will shift to the senate which, under the US constitution, is required to hold a trial on the charges.

That trial is expected to begin in January.

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

The Republican-majority chamber is all but sure to acquit the president.