“I’m too old to take a coach seat on a kamikaze flight,” he said.

Mr. Sondland, the ambassador to the European Union and a major figure in the Ukraine saga, apparently fit Mr. Luskin’s bill. So far, he is the only witness who dealt repeatedly and directly with Mr. Trump and who is cooperating with the House inquiry into whether the president abused his power in pressing Ukraine’s president to announce investigations that could benefit Mr. Trump personally.

Even with Mr. Luskin by his side, the ambassador fared poorly with House investigators when he first testified behind closed doors on Oct. 17. Testimony by subsequent witnesses exposed holes in Mr. Sondland’s account of his role in the Ukraine affair. He was forced to amend it in writing.

Andrew Weissmann, a former prosecutor on the special counsel’s team that investigated Russian interference in the 2016 election, said he could imagine the advice that Mr. Luskin gave Mr. Sondland before he returned to the witness table.

“There is no doubt in my mind that he had a tough-love conversation with him that said, ‘You are coming clean now, or else, a) I’m not representing you or, b) all bets are off,’” Mr. Weissmann said on MSNBC. “Basically you already had one bite at the apple and you saw what happened. There are documents out there. There are credible witnesses out there. You better tell the truth.”

Mr. Sondland remained fuzzy about many details of the Ukraine saga on Wednesday, the result, he said, of the administration’s refusal to release records to him. But he was adamant about two crucial points: that he acted at the president’s express direction, and that he kept top officials, including Secretary of State Mike Pompeo and Vice President Mike Pence, “in the loop.”