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.
Democrats say Mr Trump dangled $400m of US military aid and the prospect of a White House meeting for Ukrainian President Volodymyr Zelensky as bargaining chips to prod the US ally into announcing a corruption inquiry into the Bidens.
The second charge is obstructing Congress. Mr Trump, who blocked his aides from testifying, is accused of failing to co-operate with the House impeachment investigation.
The president has denied withholding US aid to benefit himself politically and maintains it was appropriate to ask Ukraine to look into alleged corruption.
What is impeachment?
Under the US constitution, a president “shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes or misdemeanours”. It is a political process, not a legal one.
The first step takes place in the House of Representatives, which is currently controlled by Democrats. Members there hold a vote to impeach, which only needs a simple majority to pass.
When this happens as expected, Mr Trump will formally have been impeached, and proceedings go on to the Senate for a trial. If two-thirds of senators then vote to convict the president, he is removed from office.
Two US presidents have been impeached – Andrew Johnson in 1868 and Bill Clinton in 1998 – but in both cases the Senate did not vote to force them from office.
Richard Nixon resigned the presidency in August 1974 when it became clear he would be impeached and ousted by Congress in the wake of the Watergate scandal.
Groups of demonstrators in favour of Mr Trump’s impeachment rallied in major cities across the US on Tuesday.
Many held signs reading “Dump Trump” and bearing the hashtag #ImpeachNow.
Surveys suggest the country is split on the process. US political website FiveThirtyEight’s collection of national polls shows just over 47% back impeachment, while 46.4% do not support it.