WASHINGTON — A dozen House Democrats are calling for President Donald Trump’s ambassador to the European Union, Gordon Sondland, to resign in the wake of revelations about his key role in the unfolding Ukraine scandal.

The calls for his ouster come as Sondland prepares to be deposed Tuesday by House committees as part of their impeachment inquiry into the president. The list includes one presidential candidate — Rep. Tim Ryan of Ohio.

Sondland, a Trump political appointee, has emerged as a central player in Trump’s bid to persuade Ukraine’s new government to commit publicly to investigate corruption and the president’s political opponents. Text messages given to Congress show Sondland and another diplomat explicitly tying Ukrainian President Volodymyr Zelenskiy’s willingness to announce an investigation to whether he would be granted a coveted White House visit.

“These text messages are deeply troubling. Mr. Sondland has lost credibility and must resign,” Rep. Gerry Connolly of Virginia said. Rep. Bennie Johnson of Mississippi agreed and said Sondland had “worked behind the scenes to carry out Trump’s wishes."

Rep. Lloyd Doggett of Texas said not only must Sondland step aside, but also everyone involved in encouraging a foreign government to interfere in U.S. politics “should be held accountable in accord with their level of complicity.”

“Yes, everyone involved or aware of Trump’s misconduct and the coverup and who failed to act should resign,” Doggett said.

Emails to Sondland and the Department of State seeking comment were not returned. The White House also did not respond to a request for comment about whether Trump intends to keep Sondland in his role.

NBC News surveyed all 235 lawmakers who make up the House Democratic Caucus about whether they believe Sondland must resign. Many of the lawmakers said they were withholding judgment until Sondland testifies Tuesday and urged him to fully cooperate with the committees, warning that failure to do so would amount to obstruction.

Several others, including Reps. Jan Schakowsky of Illinois and Yvette Clarke of New York, were leaning toward calling for his resignation — if the allegations that have emerged about Sondland are borne out.

“If these allegations are true, anyone complicit in the president’s alleged attempts to pressure a foreign leader into interfering with our election should be removed from office immediately,” Rep. Diana DeGette, D-Colo., said.

Other House Democrats who told NBC News that Sondland should resign include Reps. Denny Heck of Washington, Filemon Vela of Texas, David Price of North Carolina, Bonnie Watson Coleman of New Jersey, Anthony Brown of Maryland, Dwight Evans of Pennsylvania, Julia Brownley of California and Gwen Moore of Wisconsin.

Sondland was named by Trump's personal attorney, Rudy Giuliani, as one of two senior U.S. diplomats Giuliani coordinated with on his efforts in Ukraine. The other diplomat, former Ukraine envoy Kurt Volker, resigned amid the chaos last month. On Monday, Volker also resigned from his position as executive director of the McCain Institute, a Washington think tank, as the fallout from his role in the Ukraine story continues to mount.

Sondland’s name appears in the whistleblower report, which says that the day after Trump’s July call with Zelenskiy, Sondland and Volker met with Zelenskiy in the Ukrainian capital. The whistleblower describes Sondland as working to help the Ukrainians navigate Trump’s request for an investigation and trying to mitigate damage Giuliani was inflicting on U.S. national security.

But in text messages that Volker ultimately turned over to Congress, Sondland appears to not only be actively facilitating Trump’s goal but also shutting down a top diplomat who raised concerns.

On Sept. 9, according to the messages, acting Ambassador to Ukraine Bill Taylor tells Sondland that “as I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign.” Sondland pushes back, telling Taylor that he’s “incorrect” about Trump’s intentions and that the president has made clear “no quid pro quos of any kind.” He then advises Taylor to stop discussing the issue via text.

Sondland had no formal diplomatic experience before Trump picked him for one of the top ambassadorships. Before starting the role in 2018, Sondland was an Oregon businessman who founded a boutique hotel company, Provenance Hotels, that operates in the Pacific Northwest. He was confirmed by the Senate in June 2018 by a voice vote.