WASHINGTON — Senator Charles E. Grassley, facing what he sees as Justice Department stonewalling, is considering subpoenas to compel several witnesses to appear before the Senate Judiciary Committee and divulge what they know about President Trump’s connections to Russia and his firing of James B. Comey as F.B.I. director.

Mr. Grassley and the committee’s top Democrat, Senator Dianne Feinstein of California, have agreed that the witnesses are key to the committee’s investigation and could compel them to appear despite apparent objections by the Justice Department and the special counsel, Robert S. Mueller III.

The witnesses include two top F.B.I. officials who worked alongside Mr. Comey, James Rybicki and Carl Ghattas, as well as Paul Manafort, Mr. Trump’s former campaign chairman. Committee rules require Ms. Feinstein to sign off on subpoenas, and she has not made her position clear. But she has expressed her frustration with the delays.

“Nothing works in this area as we want it to because everything gets delayed,” Ms. Feinstein said on Thursday. “Either this happens or that happens. It’s very difficult, but we are moving along.”