Donald Trump is expected to become only the third US president in history to be impeached later by the House of Representatives.
Democratic lawmakers were ready to approve two impeachment charges against the Republican president yesterday.
Mr Trump would then face a Senate trial next month, but members of his party control that chamber and it is unlikely to remove him 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, the 45th president of the United States argued he had been treated worse than “those accused in the Salem witch trials”.
The Democratic Speaker of the House, Nancy Pelosi, called his letter “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”.
Surveys suggest the country is split on the process. US political website FiveThirtyEight’s collection of national polls shows just over 47 per cent back impeachment, while 46.4 per cent do not support it.
Members of the House are debating the matter before taking a vote on both articles of impeachment in the evening local time.
President Trump will meanwhile fly to Battle Creek, Michigan, for a “Merry Christmas” rally along with Vice-President Mike Pence.
The vote in the Democratic-controlled House is expected to fall almost entirely along party lines.
Nearly 200 Republicans are united in opposition, except for one lawmaker, Florida’s Francis Rooney, who is retiring and has not ruled out joining Democrats to impeach Mr Trump.
All but a handful of the 232 House Democrats have said they will back impeachment – about 216 votes are needed for the measure to pass by a simple majority in the lower chamber of Congress.
The yeses include most of the 31 Democratic lawmakers who represent districts won by Mr Trump in 2016.
Collin Peterson, of Minnesota, and Jeff Van Drew, of New Jersey, have said they will vote no. Mr Van Drew plans to become a Republican.
Jared Golden, of Maine, said he would vote to impeach on one charge, not both.
The House Judiciary Committee approved two articles of impeachment 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. -BBC