“A lot of the damning evidence already came out. And a lot of these witnesses are corroborating essentially the same narrative, which hasn’t changed,” said Rep. Ted Lieu (D-Calif.), a member of the House Foreign Affairs Committee.

A House Intelligence Committee source echoed that sentiment, asserting that the investigators gathering reams of evidence behind closed doors are not willing to let the process drag out, especially as the White House seeks to block the testimony of next week’s spate of high-level witnesses.

“The reality is we could fill every day of the next month with a new potential witness interview,” the source said. “Given the evidence we’ve collected so far, we think we’re ready to enter a public phase sooner than later.”

Impeachment investigators now face their most consequential moment yet: the end of the closed-door fact-finding effort in favor of a campaign to convince Americans, in full public view, that Trump deserves to be removed from office.

Speaker of the House Nancy Pelosi (center) presides over a vote by the House on a resolution formalizing the impeachment inquiry. | Win McNamee/Getty Images

That campaign began Thursday, when the House adopted a resolution governing the public phase of impeachment proceedings.

“Today, we are further down the path of our inquiry,” Speaker Nancy Pelosi said. “It’s a sad day because nobody comes to Congress to impeach a president.”

There are several high-level White House officials Democrats would like to depose but expect to resist their demands — including acting White House chief of staff Mick Mulvaney, who all but admitted a quid pro quo on television before walking it back — as well as former national security adviser John Bolton and top National Security Council lawyer John Eisenberg.

Impeachment investigators have packed four depositions onto the schedule for Monday — including Eisenberg and Brian McCormack, a senior White House budget official. But lawmakers aren’t optimistic that any of them will appear voluntarily.

Democrats could pursue legal fights against the reluctant witnesses, but they simply don't have that option if they want to keep momentum for the inquiry and wrap up the impeachment process before the 2020 presidential campaign begins in earnest.

Democrats are now likely to wind down their closed-door depositions after next week. That means the public-facing part of the impeachment inquiry could begin as soon as mid-November, when the House comes back into session after a brief recess next week.

Democrats involved in the investigation say they don’t need five, six or seven witnesses to affirm the same set of facts that Trump himself has already acknowledged, or what was provided by witnesses with firsthand knowledge. In the midst of the investigation, for example, Trump publicly urged Ukraine and China to investigate Biden — which Democrats and even some Republicans knocked as an open solicitation for foreign interference in the 2020 election.

With that in mind, leads that may provide evidence of wrongdoing by officials further down the food chain have become less compelling, investigators say.

For example, State Department official Catherine Croft told lawmakers Wednesday that she received a call earlier this year from former GOP Rep. Robert Livingston, a lobbyist for Ukrainian steel companies, urging Marie Yovanovitch’s removal as ambassador to Ukraine. Croft said she did now know why the ex-lawmaker had contacted her or who enlisted him.

Asked whether Congress should pursue Livingston’s role in the already-sprawling saga, Rep. Stephen Lynch (D-Mass.), a senior member of the House Oversight and Reform Committee, said it was probably not worth the trouble.

“We have a central purpose, and then there are some things that are not as central to the inquiry,” Lynch said. “There are a lot of interesting things on the outside edges, but I think it’s important for us to stay focused. There’s a time element here. Do you want to really keep going down that path where people on the outside are going on, or do you want to stick to the central element of this?”

Still, some Democrats are wary of shutting off the spigot of evidence too early — especially when it has flowed rapidly and with an intensity that even lawmakers did not entirely expect.

The Ukraine saga burst into public consciousness in September with the revelation of an “urgent” whistleblower complaint that an intelligence community watchdog said was “credible.” The complaint itself laid out the framework of the scandal that has engulfed Trump's presidency and has been largely corroborated by witnesses who have come forth since its existence became public.

“If you look at what we’ve done to date, each of the witnesses has had an important contribution to make,” said Rep. Sean Patrick Maloney (D-N.Y.), a member of the House Intelligence Committee. “The hearings have been very productive. They’ve served a purpose. And I understand why people are impatient, but the truth is it’s been only a few weeks and the committee has been very productive.”

Republicans, too, have argued that the witnesses with the most damning accounts are those furthest removed from Trump himself — though that case has lately become more difficult to make.

This week, two National Security Council officials, top Ukraine policy expert Alexander Vindman and top Russia policy expert Tim Morrison, corroborated facts described by William Taylor, the top U.S. diplomat in Ukraine who connected Trump directly to a plan to withhold military aid to Ukraine absent the launch of politically motivated investigations.

As the closed-door deposition phase wraps up, Democrats are setting the stage for public hearings with the witnesses who provided the most compelling evidence, such as Taylor and Vindman.

But Democrats also intend to lean heavily on the evidence provided by Trump himself: the transcript of his July 25 call with Ukrainian President Volodymyr Zelensky, in which he asked the newly elected leader to pursue an investigation of Biden.

Trump’s request came amid a pressure campaign by his personal lawyer Rudy Giuliani to persuade Ukrainians to launch the investigations. And it came at the same time Trump repeatedly postponed a White House visit for Zelensky and froze nearly $400 million in critical military assistance to Kyiv.

So far, Democrats have interviewed or deposed 12 current and former officials from the State Department, the Pentagon and the White House, most of whom defied Trump's orders and provided Democrats with potentially damaging testimony.

Though most of that testimony remains hidden from public view — a chief criticism by Republicans who oppose the impeachment process — the elements that have been released, including opening statements from several of the most explosive witnesses, have corroborated central details.

Multiple witnesses described deepening concern about Trump's posture toward Ukraine that led them to flag lawyers at the NSC. Several witnesses described a smear campaign against Yovanovitch, which was used by the president as a reason to remove her. Others described mounting concern that Trump had ordered the hold on military aid and blocked the White House visit in order to bend Zelensky to his will.

And Democrats believe the White House itself, with the publicized transcript of the July 25 phone call and Mulvaney’s own initial set of comments, has bolstered Democrats’ case that Trump abused his power.

Soon, they’ll have to sell it to the public.

