With the House taking up two articles of impeachment charging Mr Trump with abuse of power and obstruction of Congress, Mr Trump started his day by airing his grievances on Twitter.
"Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG! A terrible Thing," Mr Trump tweeted.
"Read the Transcripts. This should never happen to another President again. Say a PRAYER!"
Later, Speaker Nancy Pelosi requested that Democrats gather on the floor of the chamber "to exercise one of the most solemn powers granted to us by the Constitution".
"During this very prayerful moment in our nation's history, we must honour our oath to support and defend our Constitution from all enemies, foreign and domestic," Ms Pelosi told colleagues in a letter on the eve of the vote.
• READ MORE: Donald Trump impeachment: Why UK politicians should beware a similar fate – leader commentAs soon as the session opened, Republicans tried to halt it.
"So we can stop wasting America's time on impeachment, I move that the House do now adjourn," said Representative Andy Biggs, the chairman of the conservative House Freedom Caucus.
He forced a roll call vote - the first of several procedural efforts expected during the day to try to delay the proceedings.
It was defeated on a party-line vote. Then Republicans then tried to force a vote condemning the actions of Democratic committee leaders, based on objections to the way the Democrats conducted hearings leading to Wednesday's votes.
The president, who is expected to depart later for a rally in the election battleground state of Michigan, fired off a furious letter to Ms Pelosi on Tuesday denouncing the "vicious crusade" against him but acknowledging he was powerless to stop the expected outcome.
"When people look back at this affair, I want them to understand it, and learn from it, so that it can never happen to another president again," he wrote.
The rare undertaking to impeach a president, set to unfold over more than six hours of debate on Wednesday, has split the politicians in Congress much the way Americans have different views of Mr Trump's unusual presidency and the articles of impeachment against him.
• READ MORE: Donald Trump congratulated Boris Johnson on election win in phone callAccording to a tally compiled by The Associated Press, Mr Trump was on track to be formally charged by a House majority.
No Republicans were expected to vote for impeachment as the president's party stands firmly with Mr Trump, and the Senate, where the Republican has the majority, is expected to acquit him in a trial next year.
"Help them, and help us all," said the House chaplain, the Rev Pat Conroy, as he opened the proceedings with morning prayer.
Ms Pelosi, who warned earlier this year against pursuing a strictly partisan impeachment, nonetheless has the numbers from Democrats to approve it.
"Very sadly, the facts have made clear that the President abused his power for his own personal, political benefit and that he obstructed Congress," Ms Pelosi wrote to colleagues. "In America, no one is above the law."
Minority Leader Kevin McCarthy suggested that Republicans could try to undo the vote someday.
"Maybe a future Congress would even expunge this vote," he told Fox News, deriding the months-long impeachment proceedings as the quickest in history.
From Alaska to Florida, tens of thousands of Americans marched in support of impeachment on Tuesday evening, from a demonstration through a rainy Times Square to handfuls of activists standing vigil in small towns. They carried signs saying "Save the Constitution - Impeach!!!!" and "Criminal-in-Chief".
Mr Trump implores Americans to "read the transcript", but the facts of his July phone call with the Ukraine president were largely confirmed by witnesses in the impeachment inquiry.
Mr Trump asked Volodymyr Zelenskiy to investigate Democrats and his 2020 political rival Joe Biden.
At the time, the newly elected Ukraine leader was hoping for a coveted White House visit to showcase his standing with the US, his country's most important ally.
He was also counting on nearly 400 million dollars (£303 million) in military aid as his country confronts a hostile neighbour, Russia.
The question for members of congress, and Americans, is whether those actions, and the White House's block on officials testifying for the house investigation, are impeachable offences.
Mr Trump appeared to intend his lengthy, accusatory message less for Ms Pelosi than for the broad audience of citizens - including 2020 voters - watching history unfolding on Capitol Hill.
He accused the Democrats of acting out of "Trump Derangement Syndrome," still smarting from their 2016 election losses.
"You are the ones bringing pain and suffering to our Republic for your own selfish, personal political and partisan gain," he wrote.
Portraying himself as a blameless victim, Mr Trump compared the impeachment inquiry to the "Salem Witch Trials".
Asked later if he bore any responsibility for the proceedings, he said, "No, I don't think any. Zero, to put it mildly."