House Democrats are poised to take the historic step of impeaching President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE on Wednesday, marking what will be the most partisan vote to remove a sitting president since the country's founding.

The highly divisive vote — packed into the very last week of the legislative year — is sure to exacerbate tensions in Washington and around the country heading into a crucial election year, when echoes of impeachment could play an outsize role up and down the ballot.

The Democrats’ decision to bring impeachment to the floor caps months of House investigations into Trump’s dealings with Ukraine, and will kick the impeachment process over to the Senate, where GOP leaders are expected to stage a trial early next year that's likely to end in Trump's acquittal.

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Democratic leaders, after rejecting impeachment for much of the year, reversed course in September, following allegations from a government whistleblower that Trump had leveraged U.S. military aid and a White House meeting to press Ukrainian officials to investigate his political rivals.

The House probe led to the drafting of two impeachment articles charging Trump with abusing power and obstructing Congress. Wednesday’s votes on those measures — while energizing the party’s liberal base — pose a political risk for a number of centrist Democrats, particularly the 30 lawmakers representing districts carried by Trump in 2016.

Even so, those same vulnerable Democrats have overwhelmingly dismissed the political threat and emerged, one by one, in recent days to announce their support for both articles, all but ensuring easy passage when they hit the floor.

“It became clear to me that he did, in fact, withhold critical military aid from a security partner in order to investigate — or attempt to have a foreign power investigate — an opponent of his in our democratic election process,” freshman Rep. Mikie Sherrill Rebecca (Mikie) Michelle SherrillHillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll House passes legislation to boost election security research Lawmakers call for bipartisan push to support scientific research MORE (D-N.J.), a Navy veteran from a Trump-won district, said Tuesday, just after announcing her support for the articles.

Democratic leaders are expecting only two defectors — Reps. Collin Peterson Collin Clark PetersonKate Schroder in Ohio among Democratic challengers squelching GOP hopes for the House The Hill's Campaign Report: 19 years since 9/11 | Dem rival to Marjorie Taylor Greene drops out | Collin Peterson faces fight of his career | Court delivers blow to ex-felon voting rights in Florida Peterson faces fight of his career in deep-red Minnesota district MORE (Minn.) and Jefferson Van Drew (N.J.) — to join Republicans in opposing both articles. A third Democrat, Rep. Jared Golden (Maine), said he will split his votes, supporting the abuse of power charge but not obstruction of Congress.

Both articles are expected to pass, making Trump just the third president to be impeached in U.S. history — and the first to run for reelection afterward.

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Democratic leaders are not whipping votes for support, calling the impeachment vote one of conscience. They’re dismissing the notion that vulnerable Democratic supporters will face a backlash on Election Day.

“I think the greatest protection these members will have — all members will have — is that they vote their conviction and the Constitution, not their politics,” House Majority Leader Steny Hoyer Steny Hamilton HoyerCentrist Democrats got their COVID bill, now they want a vote House to vote on resolution affirming peaceful transition of power On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline MORE (D-Md.) said Tuesday. “The truth is going to be the best advocate for these members."

Republicans, who have been pushing to maximize Democratic defections, watched this week as the number crept lower and lower with each new endorsement. Still, Republicans notched a victory when Van Drew, after huddling with Trump at the White House, signaled he would soon abandon the Democratic Party.

In a cryptic conversation with reporters on Tuesday, however, Van Drew declined to say not only when he would make the jump, but if he would at all.

“I have not made a decision that I'm willing to share with anybody,” he said. “I am where I am. And you'll hear in a relatively short period of time.”

House Republicans, meanwhile, appear to have maintained a strong firewall in opposing the two articles heading into the vote, with no GOP members visibly supporting impeachment and only a handful publicly denouncing Trump’s contacts with Kyiv.

Only one Republican, Rep. Francis Rooney Laurence (Francis) Francis RooneyThe Hill's Convention Report: Democrats gear up for Day Two of convention The Hill's Morning Report - Presented by Facebook - Gohmert tests positive; safety fears escalate on Capitol Hill Pelosi to require masks on House floor MORE (Fla.), has declined to say how he’ll vote. But on Tuesday he criticized Democrats for moving too quickly through the process, suggesting he’ll join his GOP colleagues when the articles come to the floor.

“I've gone from being open and wanting to hear all the facts to being extremely frustrated and disappointed that the Democrat process didn't slow down,” Rooney, who’s retiring at the end of this Congress, told CNN.

The near-unanimity expected from both sides on Wednesday means Trump's impeachment will be much more partisan than that targeting former President Clinton in the late 90s.

In the Clinton impeachment vote, five Democrats split with their party and joined Republicans in supporting three of the four articles of impeachment, while one Democrat supported all of the charges.

Meanwhile, two articles of impeachment against Clinton failed in the House because of lack of GOP support. Eighty-one Republicans voted against the last abuse of power charge, and 28 Republicans voted against the second article charging Clinton with perjuring himself in a civil case — defeating both articles before they reached the Senate.

Many who lived through the Clinton impeachment period say it was the beginning of the end of bipartisanship in Congress.

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The dramatic move to impeach Trump comes less than a year after Democrats regained the House majority and after months of investigations into Trump’s conduct, beginning first with investigations into former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's report on Russia's election interference and then into Trump's actions related to Ukraine.

While both sides have alleged that the other party is seeking to undermine the upcoming presidential election — with Democrats pointing to Trump’s request for a Biden probe and Republicans pointing to the impeachment vote ahead of an election year — Republicans also say the ballot box will serve as a faster judge on the impeachment inquiry than those writing the history books.

“This is nothing more than an illegal, partisan attempted coup that will, based on recent sentiment, badly fail at the voting booth. You are not just after me, as President, you are after the entire Republican Party,” Trump wrote in a scathing letter to Speaker Nancy Pelosi (D-Calif.) on Tuesday.

“History will judge you harshly as you proceed with this impeachment charade,” the president added, describing efforts to remove him as an “impeachment fantasy.”

Democrats have dismissed the GOP allegations, saying their choice is a matter of principle, not politics.

“Very sadly, the facts have made clear that the President abused his power for his own personal, political benefit and that he obstructed Congress as he demanded that he is above accountability, above the Constitution and above the American people. In America, no one is above the law,” Pelosi wrote Tuesday in a "Dear Colleague" letter ahead of the high-stakes votes.

“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.”

Scott Wong contributed.