FBI Director James Comey has promised to tell two key senators the scope of the agency's investigation into President Donald Trump's ties to Russia on Wednesday, one of the senators says.

Sens. Lindsey Graham, R-S.C., and Sheldon Whitehouse, D-R.I., are the chair and ranking member of the Judiciary Committee's Subcommittee on Crime and Terrorism, respectively, and are leading one of three congressional investigations into Russia's attempts to influence last year's election. Their subcommittee has oversight of the FBI.

Whitehouse on Tuesday told CNN that he and Graham met privately with Comey on March 2 -- the same day Attorney General Jeff Sessions recused himself from any such investigations. Whitehouse said he and Graham were assured that Comey would confirm whether an investigation exists and, if so, the scope of it.

The FBI has not separately confirmed Comey will respond to the senators' request by Wednesday.

The subcommittee is set to hold a hearing Wednesday afternoon on the "modus operandi and toolbox of Russia and other autocracies for undermining democracies throughout the world," and is expected to deal extensively with the U.S. intelligence community's unanimous conclusion that the Kremlin intended to interfere in the 2016 U.S. presidential election in order to help Trump defeat Democratic nominee Hillary Clinton.

"So we said look, we're going to go ahead with our hearing on the 15th, just let us know by then," Whitehouse told Politico on Tuesday. "So I think he implicitly confirmed that they are looking at this and he said he'd get us a clearer explanation by tomorrow. That was satisfactory to both of us."

The two senators have expressed their frustrations that the FBI has not been more forthcoming with details on its investigation.

Earlier Tuesday, Graham told reporters the FBI is "about to screw up big time" in ignoring his subcommittee's requests for information, particularly with regard to Trump's allegations that then-President Barack Obama ordered the wiretapping of Trump Tower.

Last week, he and Whitehouse sent Comey a letter asking for details – "any warrant applications and court orders" – but have yet to hear a response.

"If they don't honor this request and give us an answer, then I would say that we need a joint select committee because the regular order is not working," Graham told reporters Tuesday. "They're about to screw up big time if they keep running to the Intel Committee and not answer that letter."

Whitehouse said he was optimistic Comey would offer a "clearer explanation."