WASHINGTON • President Donald Trump assailed the House impeachment probe and accused Democrats of trying to overturn his election, as lawmakers prepared for a final day of debate followed by a vote that is expected to make Trump the third U.S. president to be impeached.

Potential drama over the outcome of Wednesday’s planned vote evaporated after Democratic fence-sitters largely lined up against Trump. Nearly all of the 31 Democrats who represent districts Trump won in 2016 have said they would support impeachment, giving Democrats enough support to pass the articles without Republican votes.

“We do have this responsibility to hold an out-of-control president accountable,” said Democratic Caucus Chair Hakeem Jeffries of New York. “We’re not going to shirk from that constitutional responsibility.”

The Democratic-controlled House will vote on two articles of impeachment. The first alleges that Trump abused his power as president by improperly pressuring Ukraine to launch investigations that could benefit him politically, including a probe of a leading 2020 Democratic presidential candidate. The second article alleges that Trump then obstructed the congressional investigation by blocking witnesses from testifying and withholding documents.

Trump, in a letter to House Speaker Nancy Pelosi, D-Calif., released Tuesday, called efforts to remove him from office an abuse of power that amounted to an “illegal, partisan attempted coup.” Trump, who has attacked the probe since its inception, summed up his objections in six pages of raw, personal language. “You are scarcely concealing your hatred of me,” he said, and accused the Democrats of trying to “overturn the will of the American people.”

A spokeswoman for Pelosi declined to comment on the letter. In brief remarks to reporters, Pelosi called the letter “really sick.”

Democrats say impeachment isn’t an effort to overturn the 2016 result but rather an effort to protect the next campaign from interference.

“With the 2020 elections fast approaching, we must act with a sense of urgency to protect our democracy and defend our Constitution,” said Rep. Jim McGovern, D-Mass.

Republicans objected to the process Democrats had taken to bring impeachment to the floor and said it would hurt the institution.

“To attack this president, Democrats are willing to tear down every inch of this and every other institution necessary,” said Georgia Rep. Doug Collins, the top Republican on the House Judiciary Committee.

Democrats have a majority of about three dozen seats in the House, and the caucus appeared to be largely sticking together, with only two known defections — Rep. Jeff Van Drew of New Jersey and Rep. Collin Peterson of Minnesota. On Tuesday, Democratic lawmakers in districts that Trump won by double digits backed impeachment, including Rep. Kendra Horn, who won an Oklahoma district last year in an upset, and Rep. Anthony Brindisi of New York.

No House Republicans support impeachment. Rep. Justin Amash of Michigan, an independent who left the Republican Party this year, is expected to vote with Democrats.

Trump has denied wrongdoing and reiterated in his letter to Pelosi that a controversial July phone call with Ukraine’s president was “perfect.”

In that call, Trump asked Kyiv to investigate Democratic presidential candidate Joe Biden and his son, as well as unsubstantiated allegations that Ukraine interfered in the 2016 U.S. election. A whistleblower complaint related to that call kicked off the impeachment probe in September.

If the House impeaches Trump, the matter then moves to the Senate for a trial, expected in January. The two previous presidents to be impeached, Andrew Johnson and Bill Clinton, were acquitted in the Senate. Richard Nixon resigned from office ahead of his expected impeachment.

The timeline and format of a Senate trial remained uncertain late Tuesday, as party leaders sparred.

In morning comments on the Senate floor, Majority Leader Mitch McConnell, R-Ky., dismissed requests from Democrats for new witnesses and documents in the expected Senate impeachment trial, saying it was the House’s responsibility to build the case to remove Trump from office.

His remarks came in response to a letter from Senate Minority Leader Chuck Schumer, D-N.Y., who requested the Senate subpoena four witnesses: acting White House chief of staff Mick Mulvaney; his deputy, Robert Blair; former national security adviser John Bolton; and Office of Management and Budget official Michael Duffey.

During their probe, House Democrats called all four of the witnesses Schumer is seeking, but all declined to appear after Trump directed officials not to testify.

“It is not the Senate’s job to leap into the breach and search desperately for ways to get to guilty,” McConnell said.

Schumer responded to McConnell with a floor speech. “Why is the leader, why is the president, so afraid of having these witnesses come testify?” Schumer asked.

“Senators who oppose this plan will have to explain why less evidence is better than more evidence.”

While McConnell wants a quick trial without witnesses, there is no consensus among Senate Republicans on how a trial should look. A simple majority of 51 votes could determine almost every step of the process, from rules and witness testimony to the timing of a final vote. A few GOP senators could break ranks and upset McConnell’s plans. Two-thirds of the Senate is required to remove Trump from office.

McConnell said he looks forward to meeting with Schumer so they can try to get negotiations back on track. Schumer said late Tuesday afternoon that he hadn’t heard from McConnell yet to set up a meeting.

House Majority Leader Steny Hoyer, D-Md., said that the House might not immediately send up articles of impeachment to the Senate after passage, a move that could delay a Senate trial. Some Democrats have suggested that Pelosi should delay sending over the articles to maintain some leverage over the GOP-controlled Senate.

Democrats have also held off on naming House managers, or prosecutors, for a Senate trial. House managers must be named for a trial to begin.

Reps. Jim Jordan of Ohio, John Ratcliffe of Texas and Mike Johnson of Louisiana are in talks to form the GOP impeachment team that would make presentations to the Senate, according to a person familiar with the matter. Also in discussions to possibly join that team are Collins and Rep. Elise Stefanik of New York, the person said. The team is expected to include five lawmakers.

Republicans are seeking to make Democrats pay for their votes to impeach Trump. The Republican National Committee and the Trump campaign have invested nearly $11 million in TV and digital ads in a campaign targeting about 60 lawmakers, said RNC spokesman Rick Gorka.

House Democratic leaders acknowledged the impeachment push could be putting their majority at stake.

“It may not be politically expedient” to vote on impeachment, said Rep. Katherine Clark, D-Mass., the vice chair of the Democratic Caucus.

“We don’t know how this may or may not affect the 2020 elections.”

House Rules Committee Chairman Rep. Jim McGovern, D-Mass., right, arrives for a committee hearing on the possible impeachment of President Trump, Tuesday. At left is ranking member Rep. Tom Cole, R-Okla.

Associated Press/pool