The aide is David Holmes, a source familiar with the matter told POLITICO.

Impeachment investigators announced during the hearing that Holmes, an official at the U.S. Embassy in Ukraine, is scheduled to testify behind closed doors on Friday, an indication that Democrats’ efforts to unearth new information is active even as they have begun making their public sales pitch to the American people. It is also a reflection of how rapidly the investigation has unfolded, yielding new and often startling evidence on a near-daily basis that has fueled the party’s drive toward the third presidential impeachment in U.S. history.

Democrats spent the morning eliciting testimony from Taylor and George Kent, a veteran State Department hand, that underscored deepening concern about efforts by Giuliani, Trump’s personal lawyer, to “gin up” politically motivated investigations by leaning on high-level Ukrainian officials.

Under House rules, Schiff and Daniel Goldman, a top Intelligence Committee aide and a former federal prosecutor, spent the first 45 minutes of the hearing painting a portrait of career diplomats reacting with alarm to the president’s “irregular” back-channeling to Ukraine through Giuliani in ways that appeared to counter official U.S. foreign policy and undermine national security.

In his own remarks, Schiff offered a detailed breakdown of the allegations against Trump: that he conditioned a White House meeting and nearly $400 million in military aid to Ukraine on the Eastern European country’s president, Volodymyr Zelensky, committing to an investigation of Biden and other Democrats.

Kent, a deputy assistant secretary of State, said Giuliani was aided in this effort by “some of those same corrupt former prosecutors” that State Department officials spent years trying to sideline.

“They were now peddling false information in order to exact revenge against those who had exposed their misconduct, including U.S. diplomats, Ukrainian anti-corruption officials, and reform-minded civil society groups in Ukraine,” Kent said. “In mid-August, it became clear to me that Giuliani’s efforts to gin up politically motivated investigations were now infecting U.S. engagement with Ukraine.”

Taylor said the “irregular” foreign policy included Giuliani, as well as Trump's acting chief of staff Mick Mulvaney, Energy Secretary Rick Perry, former Ukraine envoy Kurt Volker, and Sondland.

Taylor, who became the first witness behind closed doors to connect Trump directly to allegations that military aid had been withheld to pressure Ukraine, said he still believes conditioning the security assistance on the politically motivated investigations is “crazy.”

“[Sondland] described conditions for the security assistance and the White House meeting in those terms,” Taylor said. “They were dependent on, conditioned on, pursuing these investigations.”

For the next two weeks, these proceedings will be broadcast across the country and will showcase a desperately divided Congress jockeying to convince a similarly divided nation about whether Trump breached his oath of office and endangered U.S. national security.

For Democrats, the case is largely settled: Trump pressured Zelensky on a July 25 phone call to announce investigations into Biden and other Democrats.

That call came amid the broader effort by Giuliani, aided by senior State Department officials and diplomats, to demand the probes. Along the way, several of those officials warned Ukraine that a White House visit for Zelensky and hundreds of millions of dollars in military aid could hinge on succumbing to Trump’s demands.

And in an Oct. 17 press conference, Mulvaney publicly acknowledged that military aid was conditioned on Trump's demand for investigations — before walking it back several hours later.

The public proceedings marked the start of a risky, unpredictable series of open hearings that could shape Americans’ views on whether Trump deserves to be impeached.

They are also a legacy-defining test for Speaker Nancy Pelosi and Schiff, her handpicked impeachment inquiry leader. Democrats have praised Schiff, a former federal prosecutor, for leading a focused closed-door investigation of the Ukraine matter, but there remains a level of anxiety within the Democratic Caucus about the unpredictability of public hearings, which are fraught with political land mines.

Taylor and Kent were under subpoena for Wednesday’s testimony, according to an official working on the impeachment inquiry — a technical move meant to sidestep potential objections from the State Department.

Republicans sought to poke holes in the diplomats’ testimony, suggesting they were relaying second- and third-hand information rather than hearing directly from Trump. They repeatedly noted that even though military aid was delayed in July, it was released on Sept. 11 without any commitment by Ukraine to investigate the Bidens.

But that timeline ignores that Democrats had announced an investigation into the Trump-Giuliani Ukraine channel two days earlier, on Sept. 9, and that a whistleblower complaint about the Ukraine episode had been circulating inside the Trump administration for days by that point.

The Republican strategy to defend Trump was already on display before the hearing began. Three placards were assembled on easels behind the GOP seats: One quoted Rep. Al Green (D-Texas) suggesting that Trump could be reelected if he is not impeached; a second accused Schiff of knowing the identity of the whistleblower who first brought the Ukraine scandal to light; and a third was a printout of a tweet by Mark Zaid, a lawyer for the whistleblower, from January 2017, suggesting a “coup” was underway against Trump.

In his opening remarks, Rep. Devin Nunes of California, the top Republican on the Intelligence panel, harangued Democrats over what he called a “three-year-long operation” to “overturn the results of the 2016 election.”

“This is a carefully orchestrated media smear campaign,” Nunes said, describing the Ukraine controversy as a “low-rent” sequel to the investigation of the Trump campaign’s contacts with Russians.

Republicans quickly positioned themselves, effectively, as Trump’s defense counsel, seeking to legitimize the president's skepticism of Ukraine and minimize his potential role in the Giuliani-led effort. Republicans also used the hearing to pursue aspects of the Biden investigation that Trump was allegedly pushing Ukraine to look into.

GOP lawmakers and Steve Castor, a lawyer for the Republican side of the committee, pressed Kent on his testimony that he told investigators that he had expressed concerns to Biden’s office in 2015 about his son Hunter’s role on the board of Burisma, a Ukrainian energy company. Kent said he told Biden’s office that it “could create the perception of a conflict of interest.”

Trump's request for a Biden investigation has centered on Hunter Biden's relationship with that company, which had been the target of corruption investigations, though no evidence has emerged connecting the younger Biden to any of the allegations against the company.

Trump’s allies argue that his concerns about corruption in Ukraine were well-founded and that his discussion with Zelensky was appropriate. They also focused on the identity of the anonymous whistleblower who first brought to light the Ukraine controversy but has since been superseded by on-the-record testimony from diplomats and other officials describing more direct recollections of events.

Republicans have mounted a fierce rejection of the impeachment process as a “sham” that has deprived Trump of meaningful pushback and due process. Though Democrats reject this characterization, it is likely to cause flare-ups during the hearings that could become explosive flashpoints for a national audience.

