Brussels — As his name surfaced again and again this week during the start of the impeachment trial of President Trump, Gordon Sondland carried on here, almost 4,000 miles away, as if nothing unusual was underway.

Trump’s ambassador to the European Union held meetings with the E.U. official in charge of emergency management. He hosted the Korean ambassador at the U.S. mission. And, he sat with the E.U.’s Brexit negotiator. Sondland had hoped that by staying quiet and going about his business that he could stay “out of the spotlight” of the impeachment drama unfolding at the U.S. Capitol, according to a close associate.

But that spotlight was unavoidable as the colorful character at the center of the House impeachment probe reemerged this week as one of the most important figures in the Senate trial to determine whether Trump will remain in office. The House Democrats prosecuting the case and the White House lawyers defending the president have cast Sondland as their side’s “critical witness.” Together, the two have invoked Sondland’s name more than 200 times since Monday in legal briefs and statements on the Senate floor.

House Democrats — who are still unable to question officials in the president’s inner circle, such as former national security adviser John Bolton — remain dependent on Sondland’s November testimony to establish Trump’s motive and intent when he withheld aid and a coveted White House invitation last year from Volodymyr Zelensky, Ukraine’s new president.

Sondland, a business executive who gave $1 million to Trump’s inaugural committee and liked to tout his relationship with the president, testified before the House that Trump had conditioned a White House meeting on Zelensky’s announcing investigations, including one that could have been used to damage 2020 Democratic presidential hopeful Joe Biden. “Was there a quid pro quo?” Sondland testified. “The answer is yes.”

On the question of withholding aid, Sondland testified that he had come to believe that the administration was also tying almost $400 million of security assistance for Ukraine to the sought-after investigations. It added up, Sondland said, like “two plus two equals four.”

But the president’s legal team has pointed to Sondland’s testimony as evidence of the opposite of what House Democrats contend, arguing that he also told lawmakers that Trump denied that he wanted any “quid pro quo” with regard to Ukraine.

“House Democrats’ critical witness — Sondland — actually destroys their case,” Trump’s lawyers wrote in a brief filed Monday.

Trump’s team isn’t planning to end its case there concerning Sondland, according to a senior White House official. Trump’s attorneys are also preparing to impugn his credibility as the trial unfolds, saying his testimony before the House showed Sondland has a tendency to portray his uninformed opinions and presumptions as fact, which in the case of Ukraine became an echo chamber.

“So many roads come into and out of Sondland,” said the official, who like others spoke on the condition of anonymity to discuss the ambassador’s role in the trial. “Any fair-minded observer, if you looked at what the House developed in its proceedings, so much of what was claimed turned out to be Sondland had said, or insinuated, or believed, or surmised, that had been picked up and passed around by others.”

Against the spectacle of the president’s legal team undermining a sitting U.S. ambassador, Sondland remains Trump’s man in Europe.

And Sondland, for his part, is still playing along, holding on to his post in Brussels with a cheerful tenacity, according to nine associates and diplomats who have interacted with him in recent months. They say Sondland’s gamble appears to be that if he carries on, he might survive long enough to recast his brief and so far scandal-ridden record as ambassador, a role the longtime hotelier hoped would serve as a distinguished capstone to his successful career in business.

But it is unclear how long he will be kept on the job once the impeachment trial is over.

Trump has not spoken with Sondland in the two months since the explosive testimony, according to two senior U.S. officials. The two last saw each other at the White House on Oct. 2 for the Finnish president’s visit.

The decision for Trump and Sondland not to speak since the ambassador’s testimony in November might go both ways. Two acquaintances of Sondland’s said they left discussions with him in recent months understanding that Sondland had stopped calling the White House on advice of his attorneys, perhaps to avoid any perception of witness tampering.

His D.C.-based attorney, Robert Luskin, said he and his client would decline to comment. Sondland’s testimony, he said, “will speak for itself.”

Sondland’s severed tie to the White House has diminished the ambassador’s swagger, but it’s not gone, several associates said.

Sondland has continued to press Europeans to be tougher on Iranian leadership and to impose sanctions. As the two sides fight over trade, he has pushed Trump’s protectionist agenda. And as Britain finalizes its preparations to split from the E.U. next week, he has met with leaders on both sides to advocate for U.S. interests.

Some critics even say Sondland has found his stride.

“I think he’s found his feet,” said Radoslaw Sikorski, the head of the European Parliament's delegation for relations with the United States, who hosted Sondland at a meeting last week. “He has in the last year found a way of forcefully arguing for American interests and point of view without giving offense.”

Sikorski, a vocal Trump critic, said that for him and his peers, Sondland still was their main channel to Washington. “He is the U.S. ambassador, and as long he is, we assume he represents the president and the State Department,” Sikorski said. “He is well-briefed, and states it like it is.”

Others in Europe are still trying to determine Sondland’s standing.

“I don’t know how strong he really is in the U.S. right now because of the impeachment trial,” said Rasmus Andersen, a German member of the European Parliament who pressed Sondland about the U.S. political turmoil at the closed-door meeting with E.U. lawmakers last week.

Sikorski said that Sondland responded that “he wasn’t at liberty to discuss it.” As chair of the meeting, Sikorski said, he had moved the conversation along, since he saw impeachment as an internal U.S. matter.

That type of caution is a change from how Sondland operated when taking over as one of Trump’s top diplomats in the summer of 2018. Back then he eagerly sought to channel Trump, called himself a “disruptive diplomat” and touting his ability to get Trump on the phone and to speak to him as few diplomats might dare.

In a House impeachment hearing in November, State Department official David Holmes recounted listening to Sondland and Trump talk by phone while the ambassador sat in the outdoor area of a restaurant in Kyiv, Ukraine. Sondland told Trump that Zelensky “loves your ass,” Holmes said. “I then heard President Trump ask, ‘So, he’s gonna do the investigation?’ Ambassador Sondland replied that ‘he’s gonna do it,’ adding that Zelensky will do ‘anything you ask him to.’ ”

Sondland seemed amused in the House hearing when the exchange was read back to him. “Sounds like something I would say,” Sondland quipped. “That’s how President Trump and I communicate, a lot of four-letter words.”

One clear development since November is that Sondland’s involvement in a matter outside the E.U. — Ukraine policy — has all but ended, said current and former U.S. officials.

Sondland previously loomed large over U.S.-Ukraine relations, a fact that accounts for his central role in the impeachment trial.

In July, on a visit to Kyiv, Sondland told a Ukrainian broadcaster that Trump had given him “special assignments including Ukraine.”

Text messages and emails revealed in the impeachment investigation, as well as subsequently, through public records lawsuits, show that in that period, Sondland was pressing Zelensky and his aides to announce investigations into Burisma, a Ukrainian energy company where Joe Biden’s son Hunter previously held a board position, as well as into a widely discredited theory that some in Ukraine interfered in the 2016 U.S. presidential election to help Democrats.

Sondland’s unorthodox role, which included trips to Kyiv, meetings with top Ukrainian officials and one-on-one calls with Trump, gave him as much access to the president and his thinking last year as anyone who has testified in the impeachment process to date.

The difference between how House Democrats and Trump’s defense team plan to present Sondland and the significance of his testimony was on full display Wednesday as Rep. Adam B. Schiff (D-Calif.), the chairman of the House Intelligence Committee and the lead impeachment manager, in his opening statement discussed the ambassador’s role and called for Trump to be removed from office.

Schiff played a video of Sondland’s November testimony before the House in which the ambassador described carrying out what he understood to be Trump’s wishes.

“I know that members of this committee frequently frame these complicated issues in the form of a simple question: Was there a quid pro quo?” Sondland said. “As I testified previously with regard to the requested White House call and the White House meeting, the answer is yes.”

But Schiff then largely treated Sondland as a hostile witness, describing him as a foot soldier in Trump’s pressure campaign against Ukraine. He spent over 20 minutes describing Sondland as an agent of the president who “fell into line” and who warned Zelensky that bowing to Trump’s wishes on investigations would be the “price of admission” for a White House favor.

Schiff also made clear that House Democrats no longer trust that Sondland was truthful about a key conversation he claimed to have had with Trump.

Sondland has maintained that in a Sept. 9 phone call, Trump told him verbatim that he wanted “no quid pro quo” with Ukraine. Trump has held up Sondland’s depictions of the call as exculpatory.

But The Washington Post reported in November that there is no corroborating evidence such a call took place. The White House has not located a record in its switchboard logs of a call between Trump and Sondland that day, even though it has records of other calls between the two described in impeachment testimony. Also, the call would have occurred in an unusual period, between 12:47 a.m. and 5:19 a.m. in Washington.

On Wednesday, Schiff represented that the president actually used the no-quid-pro-quo phrase days earlier, on Sept. 7, during a call that had a very different thrust. In that call, Trump made clear that he in fact did want Zelensky to personally announce investigations into Trump’s political opponents, according to diplomats who spoke with Sondland at the time.

Schiff argued that the phrase was an unusual one for the president to throw out in conversation, and noted it occurred days after the White House was alerted to a whistleblower complaint detailing just such a scheme.

“Hello, Mr. President. How are you today?” Schiff said. “No quid pro quo.”

He added: “It’s the kind of thing that comes up in a conversation, if you’re trying to put your alibi out there.”