WASHINGTON (AP) — The U.S. House marched toward a historic evening vote to impeach President Donald Trump on Wednesday, with Democratic Speaker Nancy Pelosi insisting Congress must "defend democracy" by evicting him from the White House. Trump would be just the third American president to be impeached, a distinctive dark mark on his tenure.

Trump said that despite the Democrats' patriotic talk, they were actually perpetrating "an assault on America."

Pelosi invoked the the Pledge of Allegiance and the Preamble to the Constitution in arguing that the Founders' vision for a republic was threatened by the actions by Trump in the White House.

"Today we are here to defend democracy for the people,'' she said to applause from Democrats in the chamber. "I solemnly and sadly open the debate on the impeachment of the president of the United States."

Republicans swiftly came to the president's defense.

Rep. Doug Collins of Georgia warned that the Founders were just as concerned about a purely partisan impeachment, as this one is on track to become, wielded by the power of a majority party.

"This is not a solemn occasion," he mocked. "You've been wanting to to do this ever since the gentlemen was elected.''

The rare undertaking to impeach a president, unfolding over a long day of debate, has split the lawmakers in Congress much the way Americans have different views of Trump's unusual presidency and the articles of impeachment against him. Final votes were expected late in the evening.

Democrats overwhelmingly approved the rules for the debate, 228-197, with just two defections from Pelosi's ranks, an early indication of how the votes will eventually fall on the articles of impeachment.

No Republicans supported the procedural vote, but Democrats picked up backing from Rep. Justin Amash, the Michigan conservative, who left the GOP this year to become an independent over his support for impeachment.

Trump tweeted his outrage with even more capital letters and exclamation marks than usual:

"SUCH ATROCIOUS LIES BY THE RADICAL LEFT, DO NOTHING DEMOCRATS. THIS IS AN ASSAULT ON AMERICA, AND AN ASSAULT ON THE REPUBLICAN PARTY!!!!"

According to a tally compiled by The Associated Press, a clear House majority was ready to vote to impeach him. The Senate, where the GOP has the majority, is expected to acquit him in a trial next year.

"This is a democracy defining moment,"said Rep. Jim McGovern, D-Mass., the chairman of the Rules Committee, as the proceedings began. "This is about protecting our democracy."

Republican Rep. Tom Cole of Oklahoma, called it "a very sad day" with the partisan voting to come.

"Democrats have been searching for a reason to impeach President Trump since the day he was elected," he said.

One Democrat, Joe Kennedy III of Massachusetts, read a letter to his young children explaining his vote to impeach. "This is a moment you will read about in your history books," he told them.

The House chaplain, the Rev. Pat Conroy, opened the session with morning prayer. "Help them, and help us all," he said.

As soon as the session opened, Republicans tried, and failed, to halt what one called the "unfair, rigged" process. All of their efforts — to adjourn, to condemn, to delay — were soundly turned away.

The president, who was to depart later for a rally in the election battleground state of Michigan, fired off a  furious letter to Pelosi on Tuesday d enouncing the "vicious crusade" against him but acknowledging he was powerless to stop the expected outcome.

Trump implores Americans to "read the transcript," but the facts of his July phone call with the Ukraine president that sparked the impeachment inquiry have been largely confirmed by witnesses in impeachment hearings. 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 U.S., his country's most important ally. He was also counting on nearly $400 million in military aid as his country confronted its hostile neighbor, Russia.

The question for lawmakers, and Americans, is whether those actions, and the White House's block on officials testifying for the House investigation, are impeachable offenses.

On Wednesday, Republicans swiftly started offering procedural motions expected during the day to halt or delay the proceedings.

"So we can stop wasting America's time on impeachment, I move that the House do now adjourn," said Rep. Andy Biggs, R-Ariz., the chairman of the conservative House Freedom Caucus. His motion was defeated on a party-line vote. Then Republicans 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. That also went nowhere on a party-line vote.

Minority Leader Kevin McCarthy, R-Calif.,, suggested that Republicans çould 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 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."

Trump appeared to intend his lengthy, accusatory message less for Pelosi than for the broad audience of citizens — including 2020 voters — watching history unfolding on Capitol Hill.

Portraying himself as a blameless victim, as he often does, Trump compared the impeachment inquiry to the "Salem Witch Trials." Asked later if he bore any responsibility, he said, "No, I don't think any. Zero, to put it mildly."

But the House impeachment resolution says that Trump abused the power of his office and then tried to obstruct the investigation in Congress like "no other'' president in history. "President Trump, by such conduct, has demonstrated that he will remain a threat to national security and the Constitution if allowed to remain in office," it says.

Centrist Democratic lawmakers, including many first-term freshmen who built the House majority and could risk their reelection in districts where the president is popular, have announced they would vote to impeach.

Senate Majority Leader Mitch McConnell set the partisan tone for the next step, as attention will shift to the Senate which, under the Constitution, is required to hold a trial on the charges. That trial is expected to begin in January.

"I'm not an impartial juror," McConnell declared. The Republican-majority chamber is all but sure to acquit the president.

Lawmakers crossing party lines face consequences. One freshman Democrat, Jeff Van Drew of New Jersey, is indicating he will switch parties to become a Republican after opposing impeachment. Earlier this year, Michigan conservative Rep. Justin Amash left the GOP when he favored impeachment.

One new Democrat congressman, Jared Golden of Maine, said he would vote to impeach on abuse of power but not obstruction.

Hoping to dispatch with lengthy Senate proceedings, McConnell rejected Senate Democrats' push for fresh impeachment testimony and made a last-ditch plea that House Democrats "turn back from the cliff" of Wednesday's expected vote.

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Associated Press writers Laurie Kellman, Matthew Daly, Alan Fram and Aamer Madhani in Washington and David Sharpe in Maine contributed to this report.