The House will take its first vote on the impeachment inquiry of President Trump on Thursday, forcing lawmakers to go on record in support or opposition of the investigation and dictating the rules for its next phase.

House Speaker Nancy Pelosi (D-Calif.) said Monday that the vote would “affirm” the existing probe, now in its sixth week, and establish which hearings would be open and how the transcripts from witnesses who have already testified in closed sessions would be released. Pelosi said the vote also would grant due process to the president and his attorney, countering a repeated criticism by Trump that he has been treated unfairly.

“We are taking this step to eliminate any doubt as to whether the Trump administration may withhold documents, prevent witness testimony, disregard duly authorized subpoenas, or continue obstructing the House of Representatives,” Pelosi said in a letter to Democrats. “Nobody is above the law.”

The House announced it will take its first vote on the impeachment inquiry of President Trump on Oct. 31. (Reuters)

The vote is the most tangible step Democrats have taken yet in a process aimed at removing the president from office as the proceedings transform from a closed-door inquiry to a more public investigation. It will also pose a political dilemma ahead of the 2020 election for lawmakers of both parties running in closely contested districts, mindful that this vote will be viewed as a proxy vote for the larger question of impeaching Trump.

The announcement from Democrats underscored that Pelosi has the votes for the resolution; she rarely embarks on a roll call without knowing the outcome. It also suggests that the next phase of the probe — with high-profile witnesses called to testify in open session — could push a vote on whether to impeach Trump into December.

For weeks, Trump and Republicans have derided the inquiry as illegitimate while railing against the process. The White House insisted in an Oct. 8 letter that the fact that the House had not formally voted was grounds not to cooperate in the probe, although neither the Constitution nor House rules require such a vote.

Pelosi’s decision to hold a vote prompted the White House to claim vindication — with no mention of whether it would now provide the documents and witnesses the House repeatedly has requested.

“Speaker Pelosi is finally admitting what the rest of America already knew — that Democrats were conducting an unauthorized impeachment proceeding, refusing to give the president due process, and their secret, shady, closed door depositions are completely and irreversibly illegitimate,” press secretary Stephanie Grisham said in a statement.

The president’s top allies in Congress immediately criticized the vote, with Rep. Mark Meadows (R-N.C.) saying the Democrats’ appeal to transparency is “rich — considering they’ve spent weeks conducting interviews in secret, leaking their own talking points while locking down any and all information that benefits the president.”

Republicans coordinated their response and argued that the planned vote underscored that the process was flawed, according to GOP officials who spoke on the condition of anonymity to discuss the matter frankly.

Democrats said the move to formalize the impeachment inquiry marks a new phase in the investigation, reflecting the drumbeat of revelations made by witnesses who have largely corroborated the whistleblower complaint that launched the probe.

The whistleblower claimed that Trump abused his office by requesting investigations by Ukraine into his political opponents. The complaint also mentioned that nearly $400 million in congressionally appropriated military assistance and diplomatic support had been delayed. The White House has at times denied any notion of a quid pro quo.

Intelligence Committee Chairman Adam B. Schiff (D-Calif.) said the House resolution would establish that his committee will conduct the open hearings, further solidifying his role as the Democrats’ point person. Republicans have complained the probe should be led by the Foreign Affairs Committee, which has jurisdiction over the State Department.

The resolution also would ensure that any evidence or a report would be provided to the Judiciary Committee, which would vote on articles of impeachment before sending them to the full House.

“The American people will hear firsthand about the president’s misconduct,” Schiff said.

A member of the Intelligence Committee, Rep. Eric Swalwell (D-Calif.) said the vote — and the move by Democrats to open the process — makes the Republican argument “moot the minute people see a witness taking the stand.” But he was unsure it will matter in the long run to his GOP colleagues.

“I truly expect this to be like Lucy and the football in regard to their process concerns — they’ll just say this is not enough because they can’t accept” the evidence against the president, he said.

Word of the vote came as the investigation hit a new roadblock, with former deputy national security adviser Charles Kupperman failing to show for his deposition Monday. Kupperman had asked a federal judge to referee a constitutional dispute between the House and the Trump administration over whether he should testify.

Kupperman’s lawsuit threatened to slow down the probe and could affect the House’s ability to secure testimony from his former boss, onetime national security adviser John Bolton, who has retained the same lawyer as Kupperman.

Many Democrats argue that Bolton could be a star witness after several individuals testified that he was furious that Trump’s lawyer Rudolph W. Giuliani was directing a shadow policy for Ukraine.

But instead of waiting for courts to resolve the standoff, Democrats are pivoting to a new strategy and seeking to bring in lower-ranking government officials they hope can corroborate previous testimony alleging a quid pro quo. In recent days, they have fired off new letters requesting testimony from Robert Blair, a senior adviser to acting White House chief of staff Mick Mulvaney; two officials with experience at the Energy Department, Brian McCormack and Wells Griffith; and two State Department officials familiar with Ukraine policy, Catherine Croft and Chris Anderson, according to three people familiar with the proceedings. They spoke on the condition of anonymity to freely describe private deliberations.

“We are not willing to allow the White House to engage us in a lengthy game of rope-a-dope in the courts,” Schiff said Monday.

In a major victory for Democrats, an attorney for the White House’s senior director for Europe and Russia, Tim Morrison, said Monday that her client would testify if summoned by House investigators.

“If subpoenaed, my client will appear,” attorney Barbara Van Gelder said in a statement.

Morrison was a key figure in the testimony provided by Ambassador William B. Taylor, the top diplomat in Ukraine, last week. Taylor laid out a day-to-day timeline of how Giuliani took over Ukraine policy from official channels. Taylor said Morrison told him in early September that military aid for Ukraine was being held up until Ukrainian leaders agreed to publicly commit to investigate former vice president and 2020 presidential candidate Joe Biden and his family, as well as a conspiracy theory regarding the Democratic National Committee server that was hacked in 2016. Taylor later raised these concerns to senior Trump officials deputized to run Ukraine policy.

If Morrison confirms Taylor’s account, it could undercut a chief Republican complaint about the witnesses so far: that their most damning information comes from secondhand accounts.

Democratic leaders have already committed to casting the refusal by Kupperman and any others as the president obstructing Congress and thus justice — a charge that is expected to be an article of impeachment.

Democrats are not expected to call every witness who has appeared before the committee for a closed-door deposition to testify in a public hearing, but instead will feature the officials whose testimony they believe presents the most compelling narrative. Thus far, Democrats have all but settled on Taylor and former U.S. ambassador to Ukraine Marie Yovanovitch as witnesses who could sell the story to the nation.

But neither Yovanovitch nor Taylor directly interacted with the president — leaving them open to criticism from Republicans who have denounced their most damning assertions as mere hearsay.

“You know, it’s always people who have talked to people who have talked to other people who think that he might have meant this,” Meadows said Monday.

Democrats have countered that they already have direct evidence of the president’s alleged crimes in the form of the transcript of Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky, in which he asks the Ukrainian president to do him a “favor” when he raises the subject of still-undelivered military aid. They also point to Mulvaney’s public statements acknowledging that military aid to Ukraine was being held up to secure politically advantageous investigations — an assertion he later attempted to walk back in a written statement.

Tuesday, the Democrats could get their first counter to that criticism when Lt. Col. Alexander S. Vindman, a top Ukraine expert on the National Security Council, is expected to testify in the impeachment inquiry. Vindman, who would be the first person to testify who listened in on Trump’s call with Zelensky, is expected to say he was disturbed by Trump’s demand that Ukraine investigate one of his political rivals and feared it would undermine U.S. national security.

“There’s overwhelming evidence of the president’s shakedown against Ukraine. We don’t need legally to prove a quid pro quo, but the evidence is very strong as to a quid pro quo, and essentially the president’s chief of staff confessed to that on national television,” Rep. Jamie B. Raskin (D-Md.) said Monday.

Yet privately, some Democrats admit that their case would probably be bolstered further by Bolton’s testimony. Schiff said Monday he believes that the courts will rule in their favor. But the question is when.

Democratic leaders have officially committed to pursue the probe “expeditiously” — a phrase rank-and-file members and staffers translate to mean that they are committed to wrap up their proceedings by the end of 2019, so as not to let impeachment bleed into the 2020 campaign season.

To date, Democrats have been successful in the courts: In recent weeks, rulings have validated House Democrats’ demands for the president’s financial records and the grand jury information that informed former special counsel Robert S. Mueller III’s probe of Russian interference in the 2016 election. But both took months to advance through legal proceedings — time the House Democrats don’t have to wait for Bolton and his deputies.

There is nothing preventing House Democrats from sliding Bolton into the order of public witnesses at the last minute without first engaging in a private deposition. Even if he misses the House process, there is also nothing officially preventing House Democrats from introducing Bolton as a witness before the Senate, during a likely impeachment trial.

Rachael Bade, Carol D. Leonnig and Seung Min Kim contributed to this report.