WASHINGTON -- Ahead of former FBI Director James Comey's much-anticipated testimony, Sen. Mark Warner said Sunday that if President Donald Trump did in fact ask Comey to "in effect, back off of at least the investigation" into former National Security Adviser Michael Flynn, "it would be unthinkable."

In an interview with CBS News' "Face the Nation," Warner, D-Virginia, also said that "no matter what you thought about Jim Comey, the fact that the president disparaged him with comments in front of the Russians is just unacceptable." According to a summary of a meeting Mr. Trump had with Russian officials that was obtained by The New York Times, Mr. Trump said that firing Comey relieved "great pressure" on him and called Comey "crazy" and "a real nut job."

Warner, vice chairman of the Senate Intelligence Committee, announced last week with chairman Sen. Richard Burr, R-North Carolina, that Comey would be testifying before the committee on June 8.

Comey will testify before an open session first, then in a closed session in the afternoon that same day.

The fired FBI director is expected to speak about his documented interactions with Mr. Trump, in which the president allegedly asked for Comey's loyalty and asked him to drop an investigation into Flynn. Warner says, however, that Robert Mueller, who was appointed special counsel in the FBI's investigation into Russian election meddling and the Trump campaign, will have to first agree whether that Comey can look at those memos, or whether the committee can look at those memos.

"Ultimately, I think we will get a chance to look at them, and I think it's very important," said Warner.

When pressed on what key questions remain for Comey as he faces lawmakers, Warner said "I want him to reinforce one, the fact that the Russians directly intervened in our elections." Warner added, "I want to know what kind of pressure, appropriate or inappropriate, how many conversations he had with the president about this topic."

"I think Jim Comey deserves to have his, you know, in effect, day in court since the president has disparaged him so much," Warner said.

Warner said that the committee is also still sorting through any evidence of financial ties of associates of Mr. Trump, particularly as it relates to former campaign chief Paul Manafort and Flynn.

"If there are inappropriate indications of financial ties, we would look at those. We have not seen those to date. There's a lot of smoke. We have no smoking gun, but every week there's more smoke that appears. And we've got to sort through it."