A veteran ambassador ousted for political reasons she could hardly comprehend. Another seasoned diplomat, forged in multiple conflict-zone postings, who raised pointed alarms as his suspicions grew. A young refu­gee from communism who later bled in combat for his adopted homeland and feared a betrayal of democratic ideals from inside the White House.

As House Democrats embark on a new stage of their impeachment investigation of President Trump, they are pivoting from fact-finding to a campaign of persuasion — privately sketching out how they plan to use a series of blockbuster hearings with these witnesses to make the public case for Trump’s removal from office.

Key details remain unresolved and additional closed-door testimony might still be gathered, but according to interviews with more than a dozen lawmakers and aides, Democrats believe they have largely confirmed the core allegation at the heart of their inquiry — that Trump used the powers of his office, including the threat of withholding military aid, to try to force Ukraine to investigate his political rivals.

National Security Council official Lt. Col. Alexander S. Vindman testified Oct. 29 during a closed-door congressional hearing of the impeachment inquiry. (Reuters)

At the center of the Democratic case will be the transcript of Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky, in which Trump asks Zelensky to “do us a favor though,” while discussing U.S. assistance, as well as a public admission from acting chief of staff Mick Mulvaney, later retracted, confirming the White House link between aid and investigations.

But in showcasing a set of witnesses who they argue will be uniquely credible, party leaders believe they can convince the American public — and some Republicans — that Trump is unfit for office and worthy of becoming the first president in U.S. history to be removed by Congress.

“The public has heard so many conflicting statements that sometimes they don’t know who to believe. I think that after this is done, they’ll be able to focus on what happened,” said House Foreign Affairs Committee Chairman Eliot L. Engel (D-N.Y.). “And I think that we all know what happened … that the president used his power, his office, inappropriately for his own political ends against the interests of the United States.”

Still, as they move their impeachment probe into the sunlight, Democrats will confront serious risks as they seek to crack a near-complete Republican blockade. The public hearings will give the GOP an opportunity to promote their narrative: that Trump is being hounded from office by a partisan campaign that, they claim, is upending the Constitution and congressional precedent to overturn the last presidential election just ahead of the next one.

“Because it’s starting out this way, it’ll end up this way — as a partisan process, which means you’re almost certain to have a partisan vote,” said Rep. Tom Cole (R-Okla.), the ranking Republican on the House Rules Committee. “And if it comes out of here this way, I think that’s what’s going to happen in the United States Senate, as well.”

Should the Democratic-led House vote to impeach Trump, the Republican-controlled Senate would hold a trial on whether to remove him, with a two-thirds vote needed to convict.

With Thursday’s vote on the next phase, Democrats put the reins of the investigation squarely in the hands of the House Intelligence Committee — a relatively small panel whose 22 members are handpicked by each party’s top leader.

Under the rules, Chairman Adam B. Schiff and the top Republican on the panel, Rep. Devin Nunes (Calif.), will be able to engage in extended 45-minute rounds of questioning where they have the option to yield time to committee staff. Among Schiff’s aides is Daniel S. Goldman, a former federal organized-crime prosecutor in New York who has done much of the Democratic questioning in the closed-door interviews.

The new rules for public testimony sideline the two other committees — Foreign Affairs, and Oversight and Reform — who have participated in the private interviews. That means dozens of lawmakers will be on the outs, including Trump allies such as Reps. Jim Jordan (Ohio), Mark Meadows (N.C.) and Lee Zeldin (N.Y.), as well as Jordan’s staff on the Oversight panel, who have taken an especially active role in questioning the closed-door witnesses, according to people inside the room.

Republicans will have the right to suggest witnesses of their own, but Schiff and Democrats can dismiss those suggestions. Jordan said Thursday Republicans would use the impeachment hearings to counter the Democratic narrative.

“It’s easier to present a narrative when you get to call your witnesses,” he said. “But I don’t think that’s going to be allowed.”

Democrats’ pivot toward the public could begin as soon as this week, with the release of transcripts from some of the closed-door interviews conducted last month. Those transcripts could be slowly meted out, with investigators insisting they do not want to influence other witnesses by prematurely releasing others’ testimony, and the House panels are known to be pursuing almost a dozen additional witnesses.

But it’s televised testimony, not transcripts, that Democrats believe will break through the partisan divide — and those could start as early as the week of Nov. 11 and likely push a final vote into December.

Among the witnesses expected, according to lawmakers and aides, are Marie Yovanovitch, the former ambassador to Ukraine who was recalled in May amid an apparent smear campaign involving Rudolph W. Giuliani, Trump’s personal lawyer; William B. Taylor Jr., the current top diplomat in Kyiv, who became concerned over the summer about the withholding of military aid; and Lt. Col. Alexander Vindman, an Army officer who immigrated from Soviet Ukraine and was later wounded while serving as an infantry officer in Iraq. Vindman, who is detailed at the National Security Council, listened to the July 25 phone call and raised immediate concerns with superiors.

Each testified privately last month, and Democrats believe they could all give powerful testimony that not only explains Trump’s conduct but elucidates the risks it posed to national security and efforts to combat Russian aggression. They also believe their records of nonpartisan service will contrast well in the eyes of the public with Trump’s attacks on a “deep state” of unaccountable bureaucrats who he claims are seeking to undermine him.

“It turns out the deep state are deeply committed people in the military, deeply committed people in the intelligence community, deeply committed people in the State Department,” said Rep. Peter Welch (D-Vt.), an Intelligence Committee member. “They’re coming forward to advocate for enduring democratic values.”

A lawyer for Vindman, Michael Volkov, said his client will testify in public if requested. A person close to Taylor, who has returned to Kyiv, said the envoy had not received a request to appear and has made no decisions. A lawyer for Yovanovitch did not respond.

A more intriguing potential witness is former national security adviser John R. Bolton, who left the White House shortly after the phone call and who raised internal alarms of his own, according to the testimony of other witnesses.

House investigators have asked him to give a closed-door deposition Nov. 7 though it is unclear whether he will appear.

His testimony could be unpredictable, and it could be tied up in litigation for months: A former Bolton deputy, Charles Kupperman, responded to a House subpoena by asking a federal judge to determine whether he had to obey it over Trump administration privilege claims. The case is set to be argued Dec. 10.

Bolton is represented by the same attorney as Kupperman and, if he is subpoenaed, could join the same effort to seek clarity in the courts — one that prompts lengthy appeals that could delay any testimony. Schiff has said he is not eager to engage in delay tactics.

The political imperatives for Democrats are clear: A Washington Post-ABC News poll conducted last week found that Americans are almost split on whether Trump should be impeached and removed from office, with 49 percent in support and 47 percent opposed. Democrats are overwhelmingly for Trump’s removal, Republicans are overwhelmingly opposed, and independents are split almost identically to the country at large.

What has been worrisome for some Democrats is that while there was initial public shift toward impeachment after House Speaker Nancy Pelosi (D-Calif.) backed a formal inquiry in late September, largely reflecting new unity among Democrats, opinion has remained stagnant since — suggesting that the drumbeat of revelations emerging from the closed interviews have done little to convince skeptical Americans that impeachment is warranted.

Many Democrats believe the public hearings could do what the closed-door interviews did not.

Rep. Tim Ryan (D-Ohio) said there was still a significant portion of the public who stands to be moved toward impeachment — and he suggested the images of Vindman in uniform or Bolton under oath could be powerful.

“I think a lot of people who went and voted for Trump, I think they’re listening,” he said. “It’s just some of their lives are so busy that you’ve got to keep going.”

But Republicans believe that the public hearings could very well backfire on Democrats — giving the GOP equal time in front of the cameras to — if not directly attack the Democratic witnesses — undermine their testimony and make their own case that the conduct they describe does not amount to an impeachable offense.

“I think people will see there’s very little there,” said Rep. Peter T. King (R-N.Y.), a former member of the Intelligence Committee. “Almost all this testimony is about people who disagree with the policy and don’t like Giuliani making end runs. None of that’s impeachable.”

The hearings, he said, would give Republicans an opportunity to make that case: “If we don’t, it’s our fault.”

Meanwhile, a growing number of Senate conservatives have suggested acknowledging a quid pro quo while arguing that it doesn’t rise to an impeachable offense as they look to defend Trump.

Later in the process, after the House Intelligence Committee completes a report on the Ukraine allegations, the matter will be handed to the Judiciary Committee, which has traditionally handled the drafting of impeachment articles. That panel is empowered to call witnesses of its own, subject to the discretion of its chairman, Rep. Jerrold Nadler (D-N.Y.), and could incorporate matters outside of the Ukraine affairs — including allegations that Trump acted in contempt of Congress by stonewalling its investigations.

Trump and his attorney are also granted the right, subject to Nadler’s discretion, to cross-examine witnesses and suggest the consideration of additional evidence not already forwarded from other committees.

Rep. Douglas A. Collins (R-Ga.), the top Republican on the Judiciary Committee, warned Democrats Thursday that once matters reached his panel, a “cloud” would be “dropping on their heads.”

He suggested that the GOP would seek testimony from Schiff himself, whose staff had contact with the whistleblower before that person filed the complaint in August. While Democrats say that contact was limited to consultation about complaint-reporting process, Republicans have cast it as evidence of partisan malfeasance.

“Come to the Judiciary Committee, be the first witness, and take every question asked of you — starting with your own involvement with the whistleblower,” Collins said. “Folks, this ain’t over. Get ready.”

Rachael Bade, Karoun Demirjian and John Hudson contributed to this report.