He used his personal cellphone for official business and assured foreign officials they were welcome at the White House whenever they liked, she testified. On one occasion, she said, Romanian officials showed up at the White House gates with no appointment, citing Mr. Sondland.

As experts see it, his story is an object lesson in the pitfalls of handing influential foreign posts to diplomatic naïfs, while stripping oversight from the career officials at the State Department and the National Security Council.

“I told the Europeans, maybe this is the best you can expect” from the Trump administration, said Daniel Fried, a former longtime diplomat now with the Atlantic Council. No one imagined, he said, that Mr. Sondland would become a pivotal player “in this bottomless pit” of scandal.

Mr. Sondland, 62, tall and bald, is far from a typical diplomat. Foul-mouthed and unafraid to bruise egos, he craves the limelight, not policy papers and the politics of quiet persuasion that are the staples of diplomacy.

He nonetheless fits a certain mold of ambassadors: The founder of a boutique hotel chain, he landed his post after decades of work bankrolling Republican presidential candidates, including John McCain, Mitt Romney and George W. Bush and his brother Jeb Bush. In Bush family circles, he was particularly well liked.

“There aren’t many people who do it as well as he does,” David Nierenberg, an investment manager in Washington State who worked on Mr. Romney’s campaigns, said of Mr. Sondland’s fund-raising prowess. “He knew how to deliver.”

Friends said he loved the trappings of politics: ferrying presidential candidates around the northwest in his Lear jet and hosting their events at his 8,300-square-foot estate in Portland, Ore.