Mr. Schiff also said on Sunday that the whistle-blower who prompted the inquiry would testify “very soon.”

His remarks came on the same day that Thomas Bossert, Mr. Trump’s first homeland security adviser, said he was “deeply disturbed” that the president had shared a conspiracy theory about Ukraine in a July phone call with that country’s leader.

Mr. Trump had embraced the theory — which holds that Ukraine, not Russia, intervened in the 2016 election and did so on behalf of Democrats — despite repeated warnings from his staff that it had been “completely debunked,” Mr. Bossert said.

Related: Joe Biden’s presidential campaign urged TV networks to keep Mr. Giuliani off the air because of what it called misleading comments about the Biden family and Ukraine. Mr. Giuliani has alleged that Mr. Biden intervened in Ukraine to assist his son’s business interests. No evidence has surfaced to support those claims.

What’s next: Congress is on a two-week recess, which will double as a crucial time to frame the debate over impeachment and the country’s priorities.