“If we get to the point at any time where I feel we can’t do that, where there are legitimate lines of investigation that are being walled off, then I will say so,” Mr. Schiff told reporters this week.

Democrats expect the first major test of the investigation — and Mr. Nunes’s stewardship — to come on March 20, when the committee holds its first public hearing. An initial list of invited witnesses included James B. Comey, the F.B.I. director; James R. Clapper Jr., the director of national intelligence under Mr. Obama; and Sally Q. Yates, the former acting attorney general who was fired by the Trump administration after refusing to defend the president’s first travel ban.

Absent from the witness list was Michael T. Flynn, Mr. Trump’s first national security adviser, who resigned after it was revealed that he had misled Vice President Mike Pence and other White House officials about his conversations with the Russian ambassador. Though Mr. Nunes emphasized it was only an early list, he referred to Mr. Flynn as “a tangent,” portraying him as more of a victim of the intelligence leaks Mr. Nunes is seeking to investigate than a target of the inquiry.

“From everything that I can see, his conversations with the Russian ambassador, he was doing this country a favor, and he should be thanked for it,” Mr. Nunes said.

Mr. Nunes may need to tread cautiously to avoid the perception of crossing the line from impartial moderator to advocate. He has said the issue of whether Trump Tower was under surveillance during the campaign was well within the scope of the inquiry, and has denied that he added it after the president demanded that Congress investigate.

With a tight smile, Mr. Schiff said he welcomed the opportunity to disprove Mr. Trump’s accusation. But some Democrats fumed at the idea that the panel’s work could give credence to it. Should the issue not be put to rest quickly, Ms. Speier said, “it would call into question the entire investigation.”