WASHINGTON — House Democrats leading an impeachment inquiry into President Donald Trump have called on five State Department officials to appear before their committees, thrusting several veteran diplomats into the middle of a partisan clash between Congress and the White House.

Who are the diplomats at the center of the case?

Three are seasoned diplomats with years of experience under both Republican and Democratic presidents and stellar reputations among their colleagues. Two are newcomers to the State Department, one with close ties to Secretary of State Mike Pompeo and another a businessman turned ambassador who contributed to Trump's inauguration.

All five could deliver key insights into Trump's actions related to Ukraine after a whistleblower complaint alleged that Trump sought to hijack U.S. foreign policy for his own political gain, delaying military aid to Ukraine while pushing for a probe of Democratic presidential hopeful Joe Biden and his son.

Trump and his deputies have dismissed the whistleblower complaint, defended the president's phone call and actions on Ukraine, and blasted the impeachment inquiry as a purely partisan attack designed to damage the president and the administration.

Kurt Volker

Kurt Volker, perhaps the most important witness from the State Department given his rank and his role, is due to testify on Thursday. Volker stepped down as U.S. special envoy to Ukraine after his name appeared in the whistleblower report and after he was deposed to testify before House lawmakers. He served for more than two decades as a diplomat and does not have political ties to Trump. During his career, he worked on the 1995 Bosnia peace agreement and served as a legislative fellow in Sen. John McCain's office before rising to be Washington's ambassador to NATO under President George W. Bush. Volker is known for favoring a tough line on Russia and backing robust support for Ukraine. After his stint as NATO envoy, he worked in the private sector until Trump named him in 2017 as a special envoy for Ukraine negotiations, an unpaid, part-time post.

According to the whistleblower's report, Volker and the U.S. ambassador to the European Union, Gordon Sondland, flew to Kyiv and "reportedly provided advice to the Ukrainian leadership about how to 'navigate' the demands that the president had made of Mr. Zelenskiy," meaning Ukraine's President Volodymyr Zelenskiy. A day before their meetings, Trump asked Zelenskiy for "a favor" — to look into allegations against Joe Biden's son, according to a summary of the phone call between the two leaders released by the White House last week.

Volker and Sondland, and other State Department officials, spoke with the president's personal lawyer, Rudy Giuliani, in an attempt to "contain the damage" to U.S. national security from Giuliani's efforts to dig up information on Biden in Ukraine, according to the whistleblower's report.

Volker and Sondland "sought to help Ukrainian leaders understand and respond to the differing messages they were receiving from official U.S. channels on the one hand, and from Mr. Giuliani on the other," the report said.

Daniel Fried, a retired diplomat who held several senior posts during 40 years in the foreign service, said Volker's approach as described in the whistleblower report was an understandable response that many other diplomats might have undertaken. "I think Kurt is going to explain how he tried to advise Ukrainians on how to handle themselves given this difficult if not impossible situation," Fried told NBC News.

"He is relentlessly constructive," Fried said. "He was applying his skill and constructive attitude to a situation where his good instincts and skills were not enough because of the situation the president and Giuliani created."

U.S. Ambassador to Ukraine Marie L. Yovanovitch cheers new volunteers of the U.S. Peace Corps at a ceremony in Kiev, Ukraine on Nov. 30, 2017. Efrem Lukatsky / AP file

Marie "Masha" Yovanovitch

In Trump's July 25 phone call with Ukrainian President Zelenskiy, Trump refers to the former U.S. ambassador to Ukraine as "the woman" who he says is "bad news." Marie "Masha" Yovanovitch, a decorated senior diplomat who has served in three ambassadorships, most recently in Ukraine, is due to testify on Oct. 11.

She was removed abruptly from her post in Ukraine in May, months ahead of her scheduled departure, after coming under attack from right-wing media, who alleged she was hostile to the president. Her departure set off alarm bells among Democrats in Congress but the State Department said at the time her exit was "as planned."

According to the whistleblower complaint, which cited several U.S. officials, Yovanovitch's tenure was cut short because she had run afoul of the then-prosecutor general, in Ukraine, Yuri Lutsenko, and Giuliani. Lutsenko at one point alleged she had given him a "do not prosecute" list. The State Department has said the assertion was an outright fabrication and Lutsenko himself later walked back his comments.

Her former colleagues describe her as one of the State Department's most talented and conscientious diplomats, and that it would be totally out of character for her to engage in partisan politics.

"There are some foreign service officers who are willing to go out on a limb if they think it's important," said one former senior diplomat who worked with Yovanovitch and helped shape U.S. policy on Russia and former Soviet republics. "She's one who always stays within her instructions. The charges against her are preposterous given the type of person she is. She is an innocent victim of political machinations in two capitals."

In their interview with Yovanovitch, lawmakers likely will ask the diplomat if she or the embassy staff were asked to assist Giuliani in any way, and what her response was.

During her tenure, Yovanovitch was outspoken in her calls for Ukraine to tackle corruption, a stance in keeping with U.S. policy over successive administrations.

After Yovanovitch gave a tough speech in March urging the government to sack a senior anti-corruption official, she came under fire from Lutsenko, conservative voices in the U.S. and the president's son, Donald Trump Jr.

"I think she was a minor player in this whole burgeoning problem, who was taken out because the people in Ukraine who were useful to Giuliani and President Trump had it in for her. And they had it in for her because she was doing her job," the former diplomat said.

Yovanovitch is currently a State Department fellow at Georgetown University where she is teaching graduate students a class entitled "Policy Analysis on Ukraine" for the fall semester.

Diplomat George Kent. U.S. State Dept

George Kent

George Kent is a career foreign service officer who was the No. 2 ranking diplomat at the U.S. embassy in Ukraine from 2015 to 2018, serving under Ambassador Yovanovitch for much of that time. He is currently the deputy assistant secretary in the European and Eurasian Bureau overseeing policy on Ukraine and five other countries. Lawmakers will likely want to know if he is one of the State Department "officials" referred to by the whistleblower as taking part in conversations with Ukrainian officials about how to manage inquiries from the president's lawyer.

Fluent in Russian, Ukrainian and Thai, Kent joined the diplomatic corps in 1992. Former colleagues say Kent is a brilliant diplomat who had an excellent understanding of Ukraine. One former senior U.S. official who worked with Yovanovitch and Kent said the Ukraine affair has placed them in an uncomfortably public position.

"These people like Masha and George, they just want to keep their heads down and do their job. They don't want any part of the media limelight. They hate this stuff. They were just trying to help Ukraine to become less corrupt and more stable," the former official said.

Gordon Sondland, the United States Ambassador to the European Union, addresses the media during a press conference at the US Embassy to Romania in Bucharest on Sept. 5, 2019. Daniel Mihailescu / AFP - Getty Images file

Gordon Sondland

U.S. ambassador to the European Union Gordon Sondland, who House Democrats have asked to testify on Oct. 10, is a political appointee and long-time Republican donor without prior diplomatic experience.

According to the whistleblower report, Sondland met at least twice with Ukrainian officials, along with Volker, the special envoy to Ukraine, "to help Ukrainian leaders understand and respond to the differing messages they were receiving from official U.S. channels on one hand and from Mr. Giuliani on the other."

Two former U.S. officials say he was supportive of Volker's diplomacy on Ukraine, and favored lending U.S. assistance to Kyiv to counter the threat posed by Russia and pro-Russian forces in eastern Ukraine.

A hotel mogul, Sondland is director of the Aspen Companies, a private equity firm, and CEO of Provenance Hotels, a network of 14 boutique hotels. Among his most prized possessions is a first edition of Ayn Rand's novel "Atlas Shrugged," signed by the author, which he gave to his wife, Katy Durant, according to a 2018 interview.

Sondland's support for President Trump was not unqualified. In July 2016, Sondland was listed by the RNC as one of more than 80 bundlers for Trump. But one month later, Sondland publicly pulled his support after Trump criticized Gold Star parents Khizr and Ghazala Khan, whose son was killed in Iraq.

After Trump was elected, Sondland donated $1 million to Trump's 2017 inaugural committee, according to the Center for Responsive Politics. He was nominated as ambassador to the E.U. just over a year later.

Two weeks before the Ukraine revelations, Sondland discussed his role in negotiating trade relations between the E.U. and the U.S. and defended Trump's approach to Europe in an interview with Politico. Asked if Trump was good company, he said, "He's a hell of a lot of fun."

T. Ulrich Brechbuhl. U.S. State Dept

T. Ulrich Brechbuhl

Soon after taking the helm as secretary of state in May 2018, Mike Pompeo named an old classmate from West Point, T. Ulrich Brechbuhl, to be State Department counselor, a senior adviser role. Pompeo and Brechbuhl's shared experiences go back decades. Both graduated from the same West Point class in 1986, and both earned higher degrees from Harvard. Brechbuhl's degree was in business, while Pompeo went to the law school.

Brechbuhl, born in Switzerland and fluent in four languages, later became a business partner with Pompeo in Kansas, helping him found Thayer Aerospace, a firm that reportedly included investment backing from the Koch brothers.

He has kept a low public profile as Pompeo's adviser, but given his close ties and access to the secretary, lawmakers likely will be asking him what he knows about the administration's dealings with Ukraine over the past year.

When he started his post, Brechbuhl's priorities included helping to fill numerous vacant top leadership positions and "get our team staffed up," then-State Department spokesperson Heather Nauert said at the time.

As counselor, he reports directly to the secretary, providing "strategic guidance" on foreign policy, conducts special international negotiations and handles "special diplomatic assignments," according to the State Department.

Brechbuhl was a guest at a June 4 dinner hosted by Sondland, Trump's ambassador to the E.U., at which Volodymyr Zelenskiy, the president of Ukraine, was also in attendance, as was Trump's son-in-law, Jared Kushner, according to photos released from the U.S. mission to the E.U.

None of the five diplomats responded to requests for comment from NBC News.

The documents Congress wants

Apart from testimony from the deposed State Department officials, House congressional committees conducting the impeachment inquiry have subpoenaed Pompeo for an extensive list of documents. Pompeo has pledged to produce the documents on Friday.

Those documents include records related to the July 25 call between Trump and Zelenskiy, as well as a list of State Department officials who "participated in, assisted in preparation for, or received a readout," and any copies of a transcript that is in the State Department's hands. In addition, the congressional committees are asking for records referring to Giuliani, and those that refer or relate "in any way" to the suspension of security assistance to Ukraine.