A top U.S. diplomat who raised concerns about the Trump administration’s decision to withhold aid to Ukraine is slated to testify Tuesday behind closed doors as House Democrats press ahead with their impeachment inquiry.

Democrats are hoping to receive powerful testimony from William Taylor, a career foreign service official who has worked as chargé d’affaires for Ukraine since June. His deposition is the latest in a string of appearances by civil servants and political appointees providing details about President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE’s contacts with Ukraine.

Taylor is considered a high-value witness because he appeared to threaten to quit his post after describing a “nightmare scenario” in text messages with two other U.S. diplomats — Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE, then Trump’s special envoy to Ukraine, and U.S. Ambassador to the European Union Gordon Sondland.

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According to messages Volker turned over to the House, Taylor raised concerns that Ukraine would make good on providing an unspecified “interview,” but that Trump would renege on almost $400 million in military aid.

“The nightmare is they give the interview and don’t get the security assistance,” Taylor texted to Volker and Sondland, according to transcripts released by Democrats. “The Russians love it. (And I quit).”

The next day, Taylor warned Sondland that Trump’s previous decision to withhold the aid had already strained relations between the two allies to the benefit of Russia, which launched an incursion into Ukraine in 2014 and annexed eastern parts of the country.

“The message to the Ukrainians (and Russians) we send with the decision on security assistance is key,” Taylor wrote. “With the hold, we have already shaken their faith in us. This is my nightmare scenario.”

That fear came as Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, the president’s personal lawyer, was seeking a statement from Ukrainian President Volodymyr Zelensky that addressed both Burisma, a Ukrainian energy company that employed former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE’s son and a debunked conspiracy theory related to interference in the 2016 U.S. presidential election.

The text messages provided by Volker also allude to phone calls Taylor had with Volker and Sondland about Ukraine policy. Democrats are eager to learn more about those discussions.

“He was interacting with Volker and Sondland about this alleged scheme to pressure the Ukrainian government to assist with a domestic, political initiative,” said Rep. Raja Krishnamoorthi Subramanian (Raja) Raja KrishnamoorthiCDC causes new storm by pulling coronavirus guidance Democratic chairman says White House blocked Navarro from testifying Democrats urge CDC to update guidance to encourage colleges, universities go tobacco-free MORE (D-Ill.), who sits on two of the three House committees leading the impeachment probe.

“And interestingly, his text messages are very revealing into what he thought was happening. So I think it is very important to hear from him [and] educate us about what he was thinking when he wrote those messages,” he said.

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Democrats hope Taylor’s stated concerns about the administration’s policy toward Ukraine, combined with his tenure as a career official, will make him a helpful witness as they seek to determine whether Trump withheld aid as leverage to get Zelensky to open an investigation into Biden, one of his top 2020 political opponents.

While Democrats argue that Trump used the aid as leverage, Republicans contend that witnesses like Sondland — and even Volker’s text messages — back up Trump’s assertion that there was no quid pro quo.

Taylor’s appearance on Tuesday will follow similar depositions from career State Department officials in recent weeks, including Fiona Hill, a former special assistant to the president; Michael McKinley, a former staffer to Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE; and Deputy Assistant Secretary of State George Kent.

Democrats have indicated they view the testimonies of career officials as more credible than those of the political witnesses, including Sondland.

Taylor has amassed a long record of public service after graduating from West Point and Harvard. In 2006, he served as U.S. ambassador to Ukraine for three years. After Marie Yovanovitch was recalled from that same post earlier this year, Taylor was tapped to serve as chargé d’affaires for Ukraine.

“My hope is that he is yet another career public servant who is apolitical who comes forward to tell us what was really happening, not what was happening from a standpoint of trying to protect yourself or trying to give a spin to the situation,” Krishnamoorthi said, in what appeared to be a jab at Sondland.

In the text message thread, Sondland, a wealthy hotel owner and GOP mega-donor, forcefully pushed back against Taylor’s comments about Ukraine aid, telling Taylor he was “incorrect.”

“The president has been crystal clear no quid pro quo’s of any kind,” Sondland later wrote. “The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelenskiy promised during his campaign.”

Taylor, however, remained skeptical.

“It’s crazy to withhold security assistance for help with a political campaign,” he wrote, in what appeared to be a reference to alleged efforts by Trump and Giuliani to get Ukraine to investigate Biden.

Sondland testified last week that he was unaware of any efforts to have Zelensky open a Biden probe, but he also said Trump insisted during a short phone call with him that there was no quid pro quo for aid, according to his prepared remarks for testimony.

Democrats walked away from Sondland’s deposition with varying opinions on his credibility, with some saying he appeared to be trying to protect his reputation from fallout from the Ukraine affair. Still, Democrats said the Trump official confirmed allegations brought forward in a whistleblower complaint about Trump pressing Zelensky to open a corruption probe into Biden.

Taylor is one of the few officials on the schedule for testimony this week. Deposition dates were shuffled around on Monday to accommodate events honoring Rep. Elijah Cummings Elijah Eugene CummingsBlack GOP candidate accuses Behar of wearing black face in heated interview Overnight Health Care: US won't join global coronavirus vaccine initiative | Federal panel lays out initial priorities for COVID-19 vaccine distribution | NIH panel: 'Insufficient data' to show treatment touted by Trump works House Oversight Democrats to subpoena AbbVie in drug pricing probe MORE (D-Md.), the House Oversight and Reform Committee chairman who died last week.

While Democrats initially had multiple witnesses planned for the week, an official working on the inquiry announced the change of plans Monday, noting that Taylor would still be testifying Tuesday and a top Defense official would follow on Wednesday.

In addition to several postponements, Democrats also have seen two more Trump administration officials vow not to comply with their subpoenas.

The postponements raise questions about whether Democrats will be able to wrap up their inquiry by the end of the year, a informal deadline members have pointed to as a possible time when they will reach a decision on impeachment.

Rep. Jim Himes James (Jim) Andres HimesMany Democrats want John Bolton's testimony, but Pelosi stays mum SEC's Clayton demurs on firing of Manhattan US attorney he would replace Democrats face tough questions with Bolton MORE (D-Conn.), a member of the House Intelligence Committee, indicated the postponements would likely lead to a much busier schedule next week.