With Gordon Sondland prepared to testify this week, Republicans backed away from complaints about secondhand information and instead offered a blunter defense: The president did nothing wrong.

House Republicans, bracing for another week of impeachment hearings, asserted on Sunday that President Trump had done nothing wrong because his plans for Ukraine to investigate his political rivals never came to fruition — even as the president complicated their efforts by attacking another witness.

On a day of back-and-forth on Twitter and the morning television talk shows that are a staple of Sundays in Washington, Speaker Nancy Pelosi invited Mr. Trump to testify before the House Intelligence Committee, while the president’s allies shifted their emphasis away from the defense they offered last week, when they stressed that witnesses had only secondhand information against him.

That argument may not work much longer, because lawmakers are about to hear from crucial witnesses who had direct contact with the president, including Gordon D. Sondland, a donor to and an ally of Mr. Trump who served as his liaison to Ukrainian officials while the president withheld — but later released — $391 million in military aid to Ukraine.

“The Ukrainians did nothing to — as far as investigations goes — to get the aid released,” Representative Jim Jordan of Ohio, one of Mr. Trump’s chief defenders, said on CBS’s “Face the Nation.” “So there was never this quid pro quo that the Democrats all promise existed.”

Ms. Pelosi, also appearing on “Face the Nation,” suggested that Mr. Trump — who is blocking key witnesses like Mick Mulvaney, his acting chief of staff, from testifying — make his case for himself, while delivering a brief lesson in Latin: “If he has information that is exculpatory, that means ex, taking away, culpable, blame, then we look forward to seeing it.”

Hours after that broadcast, Mr. Trump — who last week attacked Marie L. Yovanovitch, his former ambassador to Ukraine, on Twitter while she was testifying — unleashed an afternoon Twitter storm, lashing out at a second witness, Jennifer Williams, an adviser to Vice President Mike Pence.

Ms. Williams, a longtime State Department employee with expertise in Europe and Russia who has been detailed to Mr. Pence’s national security staff, is among those who listened to a July 25 telephone call in which Mr. Trump asked President Volodymyr Zelensky of Ukraine to “do us a favor” and investigate former Vice President Joseph R. Biden Jr. and his son Hunter.

Mr. Trump had frozen the military aid about a week before.

Ms. Williams told House investigators she thought the telephone call was “unusual and inappropriate,” adding, “I guess for me it shed some light on possible other motivations behind a security assistance hold,” according to a transcript of her deposition released Saturday evening by Democrats.

“Tell Jennifer Williams, whoever that is, to read BOTH transcripts of the presidential calls, & see the just released ststement from Ukraine,” Mr. Trump wrote, making a typographical error in the word “statement.” “Then she should meet with the other Never Trumpers, who I don’t know & mostly never even heard of, & work out a better presidential attack!”