Donald Trump is set to become the third US president in history to be impeached later by the House of Representatives.
Democratic lawmakers are ready to approve two impeachment charges against the Republican president on Wednesday.
Mr Trump is expected to face a trial in the Senate next month, but that chamber is controlled by members of his party and it is unlikely to vote that he should be removed from office.
The president has called the process an “attempted coup” and a “scam”.
In a six-page letter on the eve of the vote, Mr Trump argued he had been treated worse than “those accused in the Salem witch trials”.
The Democratic Speaker of the House, Nancy Pelosi, said his letter was “really sick”.
On Tuesday, she wrote to colleagues that impeachment was “one of the most solemn powers granted to us by the Constitution”, and called it a “very prayerful moment in our nation’s history”.
Members of the House will meet from 09:00 local time (14:00 GMT) on Wednesday. Votes on both articles of impeachment are expected between 18:30 and 19:30.
As the House prepares to vote, President Trump will fly to Battle Creek, Michigan, for a “Merry Christmas” rally along with Vice-President Mike Pence.
What are the charges?
After hours of debate, the Democratic-controlled House Judiciary Committee approved two charges against Mr Trump last week.
The first is abuse of power. It accuses the president of trying to pressure Ukraine to smear his political rival, Democratic presidential contender Joe Biden.
Mr Trump and his conservative allies have alleged without evidence that while he was US vice-president, Joe Biden encouraged Ukraine to fire its top prosecutor in order to stop him investigating a Ukrainian gas company that employed his son, Hunter Biden, as a board member.