Mr. Trump later referred to a statement by the Ukrainian foreign minister, who said last week that Mr. Sondland never explicitly linked the aid to investigations into the Bidens.

Ms. Williams is not the only witness who listened in on the call expected to testify this week. Others include Lt. Col. Alexander S. Vindman, the National Security Council’s top expert on Ukraine, who has told investigators he was so alarmed by the call that he immediately reported it to council lawyers. But Mr. Sondland’s appearance is the most anticipated.

He has given somewhat contradictory accounts. While he had said in a text message that Mr. Trump did not seek “quid pro quo’s of any kind,” he has also given a written statement to investigators in which he acknowledged that he told a top Ukrainian official that “resumption of the U.S. aid would likely not occur until Ukraine provided the public anticorruption statement that we had been discussing for many weeks.”

On Sunday, Chris Wallace , the host of “Fox News Sunday,” pressed Representative Steve Scalise , the No. 2 House Republican , on that point, asking him whether Mr. Sondland might “blow a hole” in Republicans’ defense.

“The president’s defense is that those things didn’t happen,” Mr. Scalise said, adding: “The real bottom line is he got the money. Ukraine got the mon ey.”

Later, Mr. Trump attacked Mr. Wallace on Twitter.

“@SteveScalise blew the nasty & obnoxious Chris Wallace (will never be his father, Mike!) away on Chris’s lowest rated (unless I’m on) morning show,” the president wrote. “This kind of dumb and unfair interview would never have happened in the @FoxNews past. Great job Steve!”

Both Mr. Scalise and Mr. Jordan have made such arguments before, but their remarks on Sunday suggest this will be Republicans’ core line of defense this week. What the two congressmen failed to mention, however, is that Mr. Trump released the administration’s hold on the money amid a bipartisan uproar on Capitol Hill and revelations of a whistle-blower’s report that prompted the impeachment inquiry.