Here's what to watch this week on impeachment

House Democrats are set to depose a number of new witnesses this week as part of their impeachment inquiry examining President TrumpDonald John TrumpTrump opens new line of impeachment attack for Democrats Bloomberg to spend 0M on anti-Trump ads in battleground states New witness claims first-hand account of Trump's push for Ukraine probes MORE’s contacts with Ukraine.

None of the witnesses are household names, but all could have information on the Trump administration’s activities in Ukraine — as well as Trump personal attorney Rudy GiulianiRudy GiulianiNew witness claims first-hand account of Trump's push for Ukraine probes Giuliani associate said he was on a 'secret mission' for Trump: report Pelosi: Trump tweets on Yovanovitch show his 'insecurity as an imposter' MORE’s more informal foreign policy role on behalf of the president.

The week's full list of witnesses, according to an official familiar with the impeachment inquiry, is subject to change as House Democrats continue to negotiate to bring people in for questioning.

Here's a look at some of the key figures expected to testify this week as Democrats continue to seek to build a case for impeachment.

William Taylor, chargé d'affaires for Ukraine

William Taylor is the most high-profile witness for Democrats this week.

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Taylor, who is scheduled for a deposition on Tuesday, exchanged text messages with U.S. Ambassador to the European Union Gordon Sondland and former special envoy to Ukraine Kurt VolkerKurt VolkerWhite House releases rough transcript of early Trump-Ukraine call minutes before impeachment hearing Haley: Giuliani should've been named 'special envoy' to Ukraine Overnight Energy: Perry replacement faces Ukraine questions at hearing | Dem chair demands answers over land agency's relocation | Ocasio-Cortez, Sanders unveil 0B Green New Deal public housing plan MORE, which were released publicly by Democrats, that showed he was worried the Trump administration was withholding aid to Ukraine to convince its government to conduct political investigations for Trump.

Taylor in one text appeared to threaten to quit if a possible “nightmare scenario” unfolded in which Trump reneged on providing military aid even if Ukraine’s president committed to doing an unspecified “interview.”

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

Taylor also warned in a text message one day later that the aid holdup was straining U.S. relations with Ukraine, saying: “With the hold, we have already shaken their faith in us. This is my nightmare scenario.”

A West Point and Harvard alumnus, Taylor has a long record of public service. In 2006, Taylor served as U.S. ambassador to Ukraine for three years. Following the ousting of U.S. Ambassador to Ukraine Marie Yovanovitch earlier this year, Taylor was tapped to serve as chargé d'affaires for Ukraine.

Rep. Raja KrishnamoorthiSubramanian (Raja) Raja KrishnamoorthiEx-Ukraine ambassador testifies in impeachment hearing: Five things to watch Sondland emerges as key target after Vindman testimony Ex-Trump official's refusal to testify escalates impeachment tensions MORE (D-Ill.) said the text messages between Volker, Sondland and Taylor also mention phone calls. Democrats hope to learn more about those communications during Taylor’s deposition.

Philip Reeker

Democrats are interested in talking to Reeker, the acting assistant secretary of State for European and Eurasian affairs, because other evidence suggests he might have heard about a campaign to discredit Yovanovitch.

Reeker, a career diplomat, is scheduled to testify Wednesday, a week after longtime State Department official George Kent.

The State Department inspector general provided Congress with documents earlier this month that, according to reports, showed Kent raised concerns to his superiors at State, including Reeker, that he feared there was a “classic disinformation operation” being carried out against Yovanovitch.

Reeker has nearly three decades of experience in the foreign service, according to his State Department biography. He also formerly served as the U.S. ambassador to Macedonia from 2008 to 2011 and has received multiple awards for his work in public service.

White House official Michael Duffey

Duffey is a key figure in the administration’s decision to withhold aid from Ukraine and the one political appointee in next week’s mix.

The Trump administration gave Duffey, the associate director of national security programs at the Office of Management and Budget, the authority to stall Ukraine aid after career civil servants in the White House raised concerns that they did not have the legal power to delay such funds, The Wall Street Journal reported earlier this month.

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Duffey’s role in signing off on apportionments is considered odd, the Journal reported, as sources described the responsibility as one that is typically handled by career officers who have years of experience and intimate knowledge of the funding process.

The matter of the stalled aid is being scrutinized by multiple House committees. 

Duffey, who is the second witness lined up to testify Wednesday, has also held various Pentagon posts and he has previous experience working on political campaigns, including serving as the Wisconsin state director in 2008 for the late Sen. John McCainJohn Sidney McCainDonald Trump's 2020 election economic gamble 2020 Democrats make play for veterans' votes The Memo: Democrats confront prospect of long primary MORE (R-Ariz.), according to his biography.

Defense official Laura Cooper

Democrats are slated to hear from Cooper, a top Pentagon official, on Thursday.

Cooper, the deputy assistant secretary, has a nearly two-decade career of public service, including policy areas in Russia, Ukraine and Eurasia, according to her Department of Defense biography

Cooper was originally slated to testify Friday, but her closed-door deposition was moved to the following week for unclear reasons.

A spokesman for the House Intelligence Committee declined to comment on the committee’s interest in Cooper’s testimony, but some Democrats remarked that such witnesses are helpful for building a deep understanding of how things work behind the scenes.

“As a prosecutor, what we would do is we would sort of interview as many witnesses as we can before we ask major players for interviews,” said Rep. Ted LieuTed W. LieuWyden urges FCC to secure 5G networks against cyber threats Democrat hits White House spokeswoman after Trump appointee changes testimony PETA asks DOJ to stop conducting training that harms animals MORE (D-Calif.) on Wednesday. “That's generally what you would do in an investigation."

Lt. Col. Alexander Vindman

Lt. Col. Alexander Vindman, the director for European affairs at the National Security Council (NSC), is also set to testify Thursday.

Fiona Hill, Trump’s former top Russia adviser, told House investigators that Giuliani circumvented NSC staff and other career diplomats while overseeing a “shadow foreign policy” on Ukraine that she claimed aimed to help the president politically, according to reports.

Sondland, one of the Trump officials who has come under scrutiny for his role in the Ukraine pressure campaign, has pushed back against reports of Hill’s closed-door testimony that he didn’t keep the NSC and State Department apprised of his work on Ukraine policy.

But Sondland and Hill are said to have conflicting accounts on several matters.

Additionally, Vindman makes an appearance in Sondland’s opening remarks that he delivered to House investigators Thursday. He describes Vindman as one of the few officials who attended new Ukrainian President Volodymyr Zelensky's inauguration as part of the U.S. delegation led by Energy Secretary Rick PerryRick PerryHighly irregular: Rudy, the president, and a venture in Ukraine White House releases rough transcript of early Trump-Ukraine call minutes before impeachment hearing Overnight Energy: Perry replacement faces Ukraine questions at hearing | Dem chair demands answers over land agency's relocation | Ocasio-Cortez, Sanders unveil 0B Green New Deal public housing plan MORE.

Freshman Rep. Harley RoudaHarley Edwin RoudaClub for Growth extends advertising against House Dems over impeachment Overnight Energy: Jerry Brown testifies on emissions fight | Brown presses climate action: 'Impeachment is important, but the climate is even more important' | Dems look to protect Grand Canyon from drilling Jerry Brown: 'Impeachment is important, but the climate is even more important' MORE (D-Calif.) said he believes Vindman may also have observed key meetings in which Sondland was also involved.

“So that might be why some of those witnesses are wanted,” Rouda told The Hill Friday.

Tim Morrison

Morrison, who took over Hill’s role as the senior director for European and Russian affairs on the NSC, is another individual wrapped up in efforts to connect Trump and Zelensky.

The text messages between Volker, Sondland and Taylor also mention Morrison, suggesting he was open to the idea of setting up a phone call between the two world leaders — an idea his predecessor was seeking to delay.

“I talked to Tim Morrison Fiona’s replacement. He is pushing but feel free as well,” Sondland wrote to the two other U.S. diplomats. The message came after Volker notified the text group that he had placed a “call into Fiona’s replacement” and would call Bolton “if needed” to relay the fact that Giuliani supported a Trump-Zelensky phone call.

The Washington Post also reported last month that Morrison sought to host meetings in order to determine efforts to affect Ukraine policy that went outside the normal foreign diplomacy channels.

Morrison has been asked to testify Friday, according to reports, along with Suriya Jayanti, a foreign service officer at the U.S. Embassy in Kiev.