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 Trump’s tenure at the White House.
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 from it, so that it can never happen to another president again.”
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, Trump is on track to be formally charged by a house majority.
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.
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.
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.
Portraying himself as a blameless victim, 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 Trump abused the power of his office and then tried to obstruct the investigation in congress like “no other” president in history.
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.”