WASHINGTON — As the Senate is set to vote Wednesday on the removal from office of President Trump — with acquittal all but assured — Republicans are already plotting to expunge his impeachment if they retake the House.

House Speaker Nancy Pelosi may have taunted that impeachment “will last forever,” but GOP Minority Leader Kevin McCarthy, in line to be speaker if Republicans regain the majority in the November election, doesn’t agree.

“This is the fastest, weakest, most political impeachment in history,” McCarthy told The Post on Wednesday. “I don’t think it should stay on the books.”

If McCarthy (R-Calif.) does indeed take the gavel from Pelosi (D-Calif.) in 2021, he will hold immense power to pass legislation — and a vote on expungement almost certainly would yield party-line support.

McCarthy and other Republicans say that investigating how Democrats — led by Pelosi, Intelligence Committee Chair Adam Schiff (D-Calif.) and Judiciary Committee Chair Jerrold Nadler (D-NY) — pursued their impeachment of Trump could provide the factual basis to underpin an expungement effort.

“I think [if] we take the majority, some of the key priorities for us are infrastructure, lowering prescription drugs and others. But I think when you look at what the Democrats have done, I also think we have to get to the bottom of it,” McCarthy said.

“There’s still an 18th transcript that was never released about the inspector general. It’s interesting to know, in there there was 179 pages, did Adam Schiff know the whistleblower? Did he meet with the whistleblower? I think a lot of questions are raised about whether that individual, Adam Schiff, was a fact witness.”

Republicans need to flip 18 seats to retake the House and have the majority needed to pass such a measure along party lines. With Trump on the ballot in November, they are expecting an enthusiasm boost among voters.

“We feel very, very confident that come November [voters] are going to make the right decision and Speaker Pelosi’s term as speaker of the House will not go beyond this year,” said House Republican Conference Chair Rep. Liz Cheney (R-Wyo.) on Wednesday.

The notion of expungement is likely to please Trump, who has been stung by being the third president in history to be impeached. His defense team told senators it was reasonable for Trump to ask Ukraine to investigate Democrats including former Vice President Joe Biden and his son Hunter, who scored a cushy board job at a Ukrainian energy company while his father led the Obama administration’s push to rid Ukraine of corruption. Trump denies stalling nearly $400 million in foreign aid as leverage.

Unlike other impeached presidents, Trump is poised to potentially serve another full term, increasing the political value of expungement for him and for GOP lawmakers.

Backers of the notion point to the Senate voting in 1837 to expunge a censure of President Andrew Jackson, though some legal experts believe expungement would be only symbolic.

Rep. Louie Gohmert (R-Texas) told The Post “there is precedent for doing it in a later Congress,” citing the Jackson censure vote. Gohmert said he’s thought a lot about the possibility and he’s convinced Republicans would do it.

“The president is there and I think ultimately with the things that are going to be coming out in the months ahead, it will be all the more appropriate. More and more people will see that,” Gohmert said. “So then I think by next year it will be an appropriate thing to file and do.”

Other Trump loyalists endorsed the notion in principle.

“I think there would be a groundswell of support” for expungement legislation, said Rep. Mike Johnson (R-La.), though he said it was premature to decide post-acquittal strategy.

Rep. Jim Jordan (R-Ohio) said “the president should have never been impeached in the first place” and that expungement is “a good idea.”

The idea’s been floated before. Twelve years after President Bill Clinton’s impeachment for misconduct related to an affair with White House intern Monica Lewinsky, Rep. Chaka Fattah (D-Pa.) introduced legislation disavowing the past votes. His effort went nowhere and Fattah is currently in prison for bribery, money laundering and fraud.

Jonathan Turley, the Republican-called legal scholar during the House Judiciary Committee impeachment hearing, told The Post that expungement wouldn’t mean much legally, even if it makes Republicans feel better.

“The House could hold the vote but it would be more cathartic than constitutional,” said Turley, a law professor at George Washington University. “Trump is impeached … Even if the Senate were to cancel the trial or dismiss the charges, impeachment is a historical and unavoidable fact.”

Indeed, some Republicans were skeptical of the value of expungement, even if they agreed with the sentiment.

“Certainly having it removed would be appropriate,” said retiring Rep. Mark Meadows (R-NC). “That said, I don’t know if there is a real vehicle constitutionally to get that done.”