Acting White House chief of staff Mick Mulvaney and former national security adviser John Bolton have emerged as key targets for House Democrats in their impeachment investigation of President Trump after explosive testimony about the president’s pressure on a foreign leader to investigate a political rival.

House Democrats on Tuesday began discussing the possibility of summoning both men — who would be the highest-ranking individuals to testify — as the investigation has accelerated in recent days with the cooperation of several current and former administration officials.

The actions of Mulvaney and Bolton attracted considerable attention after two witnesses testified that the acting White House chief of staff was involved in setting up a separate channel to handle diplomacy with Ukraine, which angered Bolton.

Despite stonewalling by the White House, investigators secured hours of testimony Tuesday from George Kent, the deputy assistant secretary of state responsible for Ukraine. Michael McKinley, a former senior adviser to Secretary of State Mike Pompeo, is slated to testify behind closed doors Wednesday.

“We have been bringing witnesses in at quite a furious pace,” House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) told reporters.

Last month, the White House released a rough transcript of a July 25 call in which Trump asked Ukraine to investigate former vice president and 2020 presidential candidate Joe Biden and his son Hunter at a time when U.S. military aid was being withheld from the country. A whistleblower’s complaint about the call sparked the impeachment inquiry. 

In congressional testimony, witnesses have painted a picture of a White House bitterly divided not just over Ukraine, which has long been reliant on military aid and political support from the United States as it fights Russian-backed separatists, but also over which political appointees were calling the shots on foreign policy: the experienced national security staff, or a group of Trump loyalists and the president’s personal attorney, Rudolph W. Giuliani.

In testimony Tuesday, Kent said Mulvaney organized a meeting last spring where officials decided to take Ukraine policy out of the traditional channels, putting Energy Secretary Rick Perry, U.S. Ambassador to the European Union Gordon Sondland and then-special envoy to Ukraine Kurt Volker in charge instead.

Kent told House investigators that he was instructed to “lay low,” focus on the five other countries in his portfolio, and defer to Volker, Sondland and Perry — who called themselves the “three amigos” — on matters related to Ukraine, Rep. Gerald E. Connolly (D-Va.) told reporters Tuesday.

Fiona Hill, the National Security Council’s former top adviser on Russia and Europe, told investigators Monday that Bolton was infuriated by a shadow operation being conducted by Giuliani to pressure Ukraine into digging up dirt on the president’s political rival. 

Hill said Bolton, who instructed her to raise the matter with White House lawyers, even likened Giuliani to a “hand grenade,” according to two people familiar with her testimony. Hill also testified that Bolton wanted to make clear that he was not involved and very opposed to what he called the “drug deal” between Mulvaney and Sondland, the people said.

Schiff, in consultation with House Speaker Nancy Pelosi (D-Calif.), would decide on calling Mulvaney and Bolton to testify, but several Democrats said they wanted to interview the two.

“Mulvaney has the inside understanding of why the money was withheld on the security assistance,” said Rep. Jackie Speier (D-Calif.), a member of the House Intelligence Committee, who said the two men should testify.

“They’re going to be good witnesses,” quipped Rep. Debbie Dingell (D-Mich.). “I suspect we will [need to hear from them]; we need to get the facts.”

Pelosi again Tuesday rejected calls by Trump and Republicans to hold a formal vote on an impeachment inquiry, arguing that neither the Constitution nor House rules require it.

The decision to forgo a formal vote was prompted by a combination of concerns, according to multiple Democratic aides who spoke on the condition of anonymity to describe internal discussions. A handful of vulnerable Democratic “frontliners” seeking reelection in GOP-leaning districts see some political peril in taking a vote to authorize a formal probe, while a broader swath of the caucus simply balked at the notion of having Trump and his Republican allies dictate the terms of the impeachment inquiry — an argument that ultimately swayed Pelosi.

Inside a caucus meeting Tuesday night, Rep. Anthony Brindisi (D-N.Y.), who represents a district Trump won by 15 points in 2016 and is one of only seven Democrats who have not endorsed the impeachment probe, argued to Pelosi and other leaders that the Constitution gives the House broad latitude in pursuing impeachment and that the chamber would be ill-served to “play into Republicans’ hands” and heed calls for a formal vote. “You said it perfectly,” Pelosi replied, according to three people in the room, effectively shutting down further debate.

In a news conference after the meeting, Schiff defended the inquiry against GOP attacks targeting the secrecy of the closed-door interviews and asserting that the process has been unfair to Trump. Schiff compared the private nature of the probe to the secretive special-counsel investigations that preceded the articles of impeachment against Presidents Richard Nixon and Bill Clinton. Only after those probes were largely complete did the House vote to open formal impeachment proceedings.

Schiff, moreover, argued that Republicans were simply seeking to distract from Trump’s alleged misdeeds: “They don’t want to discuss the president’s conduct. They’d rather talk process.”

Pelosi left open the prospect of holding a vote later, and the Democratic aides said that moment could come if a judge ruled against the House in court, or after the three investigative committees wrap up their initial probe and prepare to hand a case to the House Judiciary Committee for the drafting of formal impeachment articles.

“We’re not here to call bluffs,” Pelosi said. “We’re here to find the truth. . . . This is deadly serious for us.”

Discussions about next steps came as Vice President Pence, the Pentagon, the Office of Management and Budget, and Giuliani refused to cooperate with the investigation. Asked how the House planned to compel Trump allies to comply, Pelosi declined to answer.

Trump ousted Bolton last month after a rocky relationship in which the two men clashed over policy on North Korea, Iran and Afghanistan, among other issues. The president even disparaged Bolton as he left, saying he had made “some very big mistakes.”

Bolton, who initially declined to comment, said in a text: “I will have my say in due course.” He is reportedly writing a book.

Many of the revelations about Bolton’s stance were first reported by the New York Times.

According to one person familiar with Hill’s testimony, Bolton was so alarmed by the efforts of Giuliani, Sondland and Mulvaney to circumvent the National Security Council and the diplomatic corps that he dispatched her to raise the concern with White House lawyers.

The order came after Volker, Bolton, Sondland, Hill and Perry met in early July. During the meeting, Sondland blurted out to the other officials that there were “investigations that were dropped that need to be started up again” in Ukraine, according to a U.S. official familiar with the matter. The officials understood him to be referring to Burisma, a Ukrainian energy company, and Joe Biden’s son Hunter Biden, who sat on its board.

Bolton went “ballistic” after the meeting, the official said.

Hill herself got into a confrontation with Sondland over his involvement in Ukrainian affairs, according to one person familiar with her testimony, as Ukraine is not in the European Union and thus not part of his ambassadorial portfolio. Sondland said he had been put in charge by Trump, the person said — something Hill likened to the bravado of Alexander Haig, then secretary of state, who said he was in charge after a 1981 assassination attempt on President Ronald Reagan.

Sondland, who obeyed State Department orders not to show up for a planned deposition last week, is expected to testify in the House’s impeachment inquiry Thursday under subpoena. Text messages provided to the panel by Volker showed that it was Sondland who defended the president in early September, when other diplomats expressed concern that U.S. military assistance was being withheld from Ukraine to push its leaders to conduct a politically motivated investigation of Burisma.

Hunter Biden served on Burisma’s board for five years; Joe Biden is currently making a White House bid.

Trump told Mulvaney in mid-July to hold back almost $400 million in congressionally approved military aid for Ukraine. That order came the same week Hill resigned from the National Security Council; it also took place one week before Trump spoke with Ukrainian President Volodymyr Zelensky by phone, when he appeared to pressure Zelensky to investigate the Bidens and purported Ukrainian interference in the 2016 U.S. presidential election.

It is not yet clear whether the White House will attempt to block parts of Sondland’s testimony by asserting executive privilege over the president’s interactions with the ambassador. But any claim of privilege could be distinctly weakened by the White House’s decision to release a rough transcript of Trump’s call with Zelensky, according to a person familiar with the matter.  

The White House could have also missed an opportunity to claim privilege over Sondland’s conversations by failing to assert privilege over Volker’s testimony. If lawyers for Sondland — or eventually the courts — decide that means the president waived his right to claim privilege over the matters discussed in the texts Volker shared with the committees, Sondland would be able to speak freely.

The White House sent Hill letters before her deposition Monday warning her about respecting executive privilege, though it was never claimed officially to the committee. For those in government, testifying as the White House asserts privilege could come with professional consequences, but the White House has essentially no way to punish former officials — unless the information they share is classified, in which case disseminating it could be a crime. 

Greg Miller contributed to this report.