Donald Trump is set to become the third US president in history to be impeached later by the House of Representatives.
Democratic lawmakers are expected to approve two impeachment charges against the Republican president on Wednesday.
Mr Trump is scheduled 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 "witch hunt".
In a six-page letter on the eve of the vote, Mr Trump argued he had been denied his rights "from the beginning of this impeachment scam".
He has blocked top aides from testifying before lawmakers in the House and declined an invitation to appear himself.
On Tuesday, Democratic Speaker of the House Nancy Pelosi announced the vote on both charges.
Mrs Pelosi wrote to colleagues that impeachment is "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 Michigan for a "Merry Christmas" rally along with Vice-President Mike Pence.
Source: BBC
BDST: 1900 HRS, DEC 18, 2019
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