WASHINGTON — President Donald Trump is on the cusp of being impeached by the House, with a historic debate set Wednesday on charges that he abused his power and obstructed Congress ahead of votes that will leave a defining mark on his tenure at the White House.

Trump, who would be just the third U.S. president to be impeached, on Tuesday fired off a furious letter to Speaker Nancy Pelosi denouncing the “vicious crusade” against him, but he also acknowledged he was powerless to stop the expected outcome.

“When people look back at this affair, I want them to understand it, and learn for it, so that it can never happen to another president again,” he wrote.

Early Wednesday, Trump tweeted his outrage: “Can you believe that I will be impeached today by the Radical Left, Do Nothing Democrats, AND I DID NOTHING WRONG! A terrible thing.”

Pelosi, who warned earlier this year against pursuing a strictly partisan impeachment, nonetheless has the numbers from Democrats to approve it. According to a tally compiled by The Associated Press, Trump is on track to be formally charged by a House majority.

“Very sadly, the facts have made clear that the President abused his power for his own personal, political benefit and that he obstructed Congress,” Pelosi wrote to colleagues, asking them to join her on the House floor as the House convenes.

“In America, no one is above the law,” she said. “During this very prayerful moment in our nation’s history, we must honor our oath to support and defend our Constitution from all enemies, foreign and domestic.”

The rare undertaking to impeach a president, set to unfold over more than six hours of debate Wednesday, is splitting the lawmakers in Congress much the way Americans have different views of Trump’s unusual presidency and the articles of impeachment against him.

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.”

“I really believe that the Constitution is under assault,” said one protester, 62-year-old Glenn Conway, of Holly Springs, North Carolina, attending his first political rally in 30 years. “I think we have a president at this point who believes he’s above the law.”

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. 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 confronts a 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.

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.

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, as he often does, 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.”

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.

Trump “betrayed the Nation by abusing his high office to enlist a foreign power in corrupting democratic elections,” the resolution says. ”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.”

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.

Many drew on the Constitution and the intent of the country’s founders as they considered the role of Congress to conduct oversight in the nation’s system of checks and balances.

Rep. Abby Finkenauer, D-Iowa, referred to the oath she took in January as she was sworn into office as guiding her decision. She announced support for both articles of impeachment to “honor my duty to defend our Constitution and democracy from abuse of power at the highest levels.”

Republicans disagreed, firmly.

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.

“Impeachment is a political decision,” McConnell said. “The House made a partisan political decision to impeach. I would anticipate we will have a largely partisan outcome in the Senate. I’m not impartial about this at all.″

McConnell’s remarks Tuesday effectively slammed the door on negotiations for a deal proposed by the Democratic leader, Sen. Chuck Schumer, who wants to call top White House officials for the Senate trial.

Schumer’s proposal was the first overture in what were expected to be negotiations between the two leaders. Trump wants a relatively broad, perhaps showy, Senate proceeding to not only acquit but also vindicate him of the impeachment charges.

McConnell and most other GOP senators prefer a swift trial to move on from impeachment. Still, Schumer wants to hear from John Bolton, Mick Mulvaney and other current and former Trump officials who were instructed by the president not to appear for House proceedings.

“Why is the leader, why is the president so afraid to have these witnesses come testify?” asked Schumer from the Senate floor. “They certainly ought to be heard.”

Trump has promoted lawyer Rudy Giuliani’s investigation of Biden and a widely debunked theory that it was actually Ukraine, not Russia, that interfered in the 2016 election, a conspiracy-laden idea that most other Republicans have actively avoided.

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