House Democrats are bracing for a stark party-line vote Thursday on the impeachment inquiry against President Trump that will highlight the partisan nature of the probe and give him fresh ammunition to argue that he is being railroaded.

House Democratic Caucus Chairman Hakeem Jeffries said he didn’t know if his side can garner any Republican support in the vote to formally set the process for an inquiry that has been racing forward for more than a month.

“Every member of the House of Representatives tomorrow will have to decide, ‘Are we going to put principle over party, the Constitution over corruption and democracy over dereliction of duty?’ ” Mr. Jeffries told reporters. “I expect the overwhelming majority of Democrats are going to support this resolution and it’s going to pass.”

Democrats appeared to abandon hope of securing GOP defectors for the vote, as Republicans closed ranks ahead of the first test of the push to punish Mr. Trump for prodding the Ukraine president to investigate corruption involving former Vice President Joseph R. Biden, a political rival, and his son Hunter.

Mr. Trump on Wednesday called on Republicans to stop arguing about the process and “close it out” by focusing on the substance of impeachment allegations.

“Republicans are very unified and energized in our fight on the Impeachment Hoax with the Do Nothing Democrats, and now are starting to go after the Substance even more than the very [unfair] Process,” the president wrote on Twitter.

Mr. Jeffries said Democrats would call the president’s bluff.

“The only thing that we’re concerned about is presenting the substantive truth to the American people, and as long as we continue to do that, the American people will make decisions about the legitimacy of our concern,” he said.

The resolution sets ground rules for the inquiry. It gives House intelligence committee Chairman Adam B. Schiff, California Democrat, broad power to determine the course of the proceedings, including when to open hearings to the public, and directs him to deliver a report to the House Judiciary Committee that would decide on articles of impeachment.

The Judiciary Committee would then be authorized to begin its proceedings, during which the president and his counsel would be able to participate.

The new procedures also allow the intelligence committee to release deposition transcripts from the closed-door interviews that have been the mainstay of the inquiry.

Republicans, who have long objected to the closed-door hearings, to their lack of participation and the absence of due process for Mr. Trump, said the new rules change nothing.

They slammed the resolution for giving the pretense of fairness, taking particular issue with their inability to issue subpoenas without the Democrats’ approval.

“Without due process and without a fair process that respects minority rights, I do not believe the American people regard this as a legitimate process,” Rep. Tom Cole, Oklahoma Republican, said at a House Rules Committee hearing on the resolution. “Without those protections, this will be seen as just another partisan exercise.”

Attempting to preempt charges of a one-party impeachment drive, Democrats attempted to turn the tables on Republicans and blame them for any lopsided vote, accusing the GOP of blind allegiance to Mr. Trump.

“I think they really need to be careful because the long view of history will not treat them so well,” said Rep. Dan Kildee, Michigan Democrat. “This is really a party that is conceded its entire focus and its entire principle to the defense of a single person.”

The inquiry stems from a July 25 phone call in which Mr. Trump pressed Ukraine President Volodymyr Zelensky for a “favor” in investigating Mr. Biden and other corruption allegations. A whistleblower believed to be a CIA official assigned to the White House accused the president of abusing his power for personal gain, including withholding U.S. military aid from Ukraine to force the investigation.

A rough transcript of the call did not show a quid pro quo with the investigation request, but Democrats argue the threat was understood and part of an ongoing pressure campaign of “shadow” foreign policy conducted by Mr. Trump’s private lawyer Rudolph W. Giuliani.

The testimony provided so far, which has been gleaned from leaked copies of prepared opening remarks and accounts by congressional staff in the room, has largely centered on people’s opinions about what the president was doing when pressing for an investigation.

Mr. Trump wanted an investigation into allegations of corruption involving Mr. Biden and Hunter, who got a high-paying job on the board of a Ukrainian natural gas company while his father was the point man for Obama White House policy in the country, which is notorious for corruption, especially in the energy industry.

Mr. Trump also wanted Ukraine to look into a missing Democratic National Committee server that was hacked by Russia during the 2016 presidential campaign. An American cybersecurity company called CrowdStrike examined the server to probe the hack but it disappeared before it could be handed over to the FBI.

Mr. Trump subscribes to an unsubstantiated theory that the server ended up in Ukraine.

Democratic leaders until this week had resisted putting the impeachment inquiry to a vote, saying the complaints about the process masked Republicans’ inability to defend Mr. Trump’s action, which they describe as an abuse of his office for personal political gain.

The country is divided on the impeachment questions, putting Democratic lawmakers from conservative-leaning districts in a bind over the vote.

A Suffolk University poll for USA Today underscored the divide, with 37% of Americans saying the House should drop the investigation, 36% saying the House should impeach and 22% backing the probe but not impeachment.

Democrats insisted they were confident that vulnerable swing district Democrats wouldn’t be in any danger from a party-line vote.

“Public opinion has shifted strongly in favor of what we’re doing. And my Republican friends are desperately trying to find a strategy for stemming this public tide and the tide here in Congress,” said Rep. Gerry Connolly, Virginia Democrat.

At least one Democrat is likely to break with the party to vote against the inquiry.

Rep. Jeff Van Drew, who represents a conservative district in New Jersey, said the president should be investigated but any question of misconduct by the president should be settled in the 2020 election.

The closed-door investigation has been speeding forward.

House Democrats on Wednesday asked John Bolton, Mr. Trump’s former national security adviser, to testify next week in the closed-door probe. His lawyer, Charles Cooper will not let Mr. Bolton testify without a subpoena, CNN reported Wednesday evening.

They also called National Security Council lawyers John Eisenberg and Michael Ellis to testify.

Mr. Bolton has the potential to deliver the most explosive testimony regarding Mr. Trump’s dealings with Ukraine. He was involved in some of the key events with the Eastern European country.

A longtime Republican loyalist and neoconservative, Mr. Bolton could defend the president. But he was was fired by the president last month and could have an ax to grind.

The White House may try to block Mr. Bolton from testifying as it has done with previous witnesses.

Lawmakers on Wednesday heard from two State Department officials, Catherine Croft and Christine Anderson, who served under former U.S. Envoy to Ukraine Kurt Volker.

Both talked about their concerns over the role Mr. Giuliani played in Ukraine, according to copies of their opening statements obtained by The Associated Press.

Lawmakers heard testimony Tuesday from Lt. Col. Alexander Vindman of the White House National Security Council, who said he was concerned about the July 25 phone call crossing a line into partisan politics.

He also said he tried to get a few missing details from the conversation placed back into the White House’s transcript of the call, according to The New York Times.

The president criticized the lieutenant colonel, a Purple Heart recipient, for being a “Never Trumper” whose testimony didn’t hurt him.

“Yesterday’s Never Trumper witness could find NO Quid Pro Quo in the Transcript of the phone call,” the president tweeted. “There were many people listening to the call. How come they (including the President of Ukraine) found NOTHING wrong with it. Witch Hunt!”

• Lauren Meier and Jeff Mordock contributed to this report.

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