The White House insisted Trump would be busy working rather than focused on the proceedings in the House, but Trump spent his Wednesday morning tweeting, retweeting and expressing disbelief.

"Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG!," Trump wrote in one of 45 tweets posted before noon. He asked his followers to "Say a PRAYER!"

As members of the House debated the articles of impeachment, Trump's urgency appeared to escalate as he switched to all capital letters: "SUCH ATROCIOUS LIES BY THE RADICAL LEFT, DO NOTHING DEMOCRATS. THIS IS AN ASSAULT ON AMERICA, AND AN ASSAULT ON THE REPUBLICAN PARTY!!!!" he wrote.

For Trump, it was an acknowledgement of the inevitable: That he will become just the third president in US history to be impeached by the House of Representatives.

He and White House officials have tried to brush off the significance - noting there is little chance he will be convicted by the Republican-controlled Senate and removed from office - but allies nonetheless acknowledge that Trump is indeed concerned about the stain the episode will leave on his legacy.

Trump is facing two articles of impeachment. The first charges him with abuse of power by pressuring the president of Ukraine to investigate Trump's Democratic rivals while crucial US security aid was being withheld. The second charges him with obstruction of congress for stonewalling investigative efforts.

Trump had little on his public schedule on Wednesday but he was planning his own counter-programming in the evening, holding what is expected to be a highly charged rally in the battleground state of Michigan.

Otherwise, White House press secretary Stephanie Grisham said, Trump will be "working all day" as the House takes up two articles of impeachment charging Trump with abuse of power and obstruction of Congress.

"He will be briefed by staff throughout that day, and could catch some of the proceedings between meetings," Grisham said.

Vice President Mike Pence was in Michigan for a day-long bus tour before joining Trump at his Battle Creek rally. He called the expected votes a "disgrace" and said he marvelled at the president's resilience and his "determination to go forward".

He also accused Democrats of "trying to impeach this president because they can't defeat" him, predicting that Trump would once again carry the state, in part because of the energy generated by impeachment,

Trump's campaign has experienced a surge in contributions and volunteers during the proceedings and was hoping to raise another $US2 million ($A3 million) on Wednesday ahead of the votes.

Trump, for his part, had plenty to say via tweet, declaring that Democratic House Speaker Nancy Pelosi "Will go down in history as worst Speaker".

He also generously retweeted allies' comments from his favourite morning show Fox & Friends, as they sought to reassure conservative voters that Trump remains in good spirits despite the looming vote.

Kevin McCarthy, the House minority leader, and John Ratcliffe said they were among a group of GOP lawmakers who met with Trump at the White House on Tuesday evening to talk strategy about Wednesday's House session and the likely Senate trial in January.

McCarthy insisted that Trump remains focused on moving his agenda, even though his legacy will forever include impeachment.

"Anybody else, this would be traumatic for them," said McCarthy. He noted that Trump in recent days has hosted multiple White House holiday parties and will be rallying Wednesday evening. "I've never seen a man so strong. ... He is focused on what the American people need."

Australian Associated Press