Democratic lawmakers are preparing to approve two impeachment charges against the Republican president.
Mr Trump would then face a Senate trial next month, but members of his party control that chamber and are unlikely to remove him from office.
The president has called the process an “attempted coup” and a “scam”.
In a six-page letter to the Democratic Speaker of the House, Nancy Pelosi, 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”
As debate got under way in the House, President Trump was due to fly to Battle Creek, Michigan, for a “Merry Christmas” rally along with Vice-President Mike Pence.
Meanwhile, Secretary of State Mike Pompeo told reporters he would be happy to testify and produce documents for a Senate trial of Mr Trump “if that’s appropriate and required by law”.
At midday local time (1700 GMT) on Wednesday, members of the House began six hours of debate on the matter. They are expected to vote on both articles of impeachment afterwards.
In her opening remarks Ms Pelosi said it was “tragic” that the president’s “reckless actions” had led to impeachment but said he had left lawmakers with “no choice”.
“The president is an ongoing threat to our national security, and the integrity of our elections, the basis of our democracy,” she said.
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