Comey's testimony, if he appeared, would raise the stakes of the joint Judiciary and Oversight Committee investigation into the decision not to charge the 2016 Democratic presidential nominee over classified emails on her private server — and the rare step for Comey to publicly announce Clinton would not be charged — as the probe has become a proxy battle on Capitol Hill between Democrats and Republicans over special counsel Robert Mueller's investigation.
Several Republicans told CNN that Comey is a key witness for the investigation, although it's not clear how soon he would be asked to testify.
"I think that would be essential," said Rep. Matt Gaetz, a Florida Republican who has led a group of conservatives calling for Mueller to be fired. "We're going to have to find out from James Comey why the normal procedures were ignored in this case."
"I hope there's a chance at some point that we get to talk to Mr. Comey, either a deposition or under oath in a committee hearing," said Ohio Republican Rep. Jim Jordan, who last week called for Attorney General Jeff Sessions to resign over mismanagement of the Russia probe and leaks from the FBI.
Rep. Trey Gowdy, who is one of two chairmen leading the probe as head of the Oversight Committee, told CNN he expected to interview Comey "toward the end" of the investigation, declining to estimate when that would be.
"The one variable I have no control over is the availability of witnesses," Gowdy said.
Comey's last Hill testimony
If he were interviewed, Comey's testimony would mark the second time he's come to Capitol Hill since he was fired by President Donald Trump. His June 2017 public testimony before the Senate Intelligence Committee was a spectacle filled with revelations about Comey's conversations with Trump, including their discussions before his firing, as well as Comey's decision to leak memos that detailed those conversations and helped lead to Mueller's appointment.
The Judiciary and Oversight committees aren't the only ones pursuing interviews with senior FBI officials either: The House Intelligence Committee has lined up a lengthy list of FBI witnesses as part of Intelligence Chairman Devin Nunes' separate probe into the FBI and Justice Department.
Democrats have accused Republicans of using the probe into the FBI and Justice Department as a way to distract and detract from the credibility of the Mueller investigation, which is examining possible collusion between the Trump campaign and Russian officials, as well as possible obstruction of justice and financial crimes.
The timing of a Comey appearance could be dependent upon the completion of an inspector general report into Comey and other officials' handling of the Clinton investigation. The report is expected to be issued as early as March.
But at least one key Republican on the probe didn't think that would interfere because the interview may be held in a closed session.
"It doesn't have to be after the IG report — we're doing all of our hearings in a closed session anyway, so it's not an investigation that's going to be televised," said Rep. Mark Meadows, the Freedom Caucus chairman who wrote the op-ed with Jordan calling for Sessions' resignation.
Other Republicans, however, want Comey in a session open to the public.
"I'm not a big believer in members of Congress getting behind closed doors to try to find the truth," Gaetz said.
The Clinton email probe
The Oversight and Judiciary committees held their first joint interview as part of the probe into the Clinton case last month, when Deputy FBI Director Andrew McCabe testified behind closed doors for more than eight hours two days after Gowdy and Judiciary Chairman Bob Goodlatte invited him to appear.
Their December letter seeking McCabe's testimony also requested two more witnesses: FBI Chief of Staff Jim Rybicki and FBI counsel Lisa Page, who had exchanged text messages criticizing Trump and other politicians with FBI agent Peter Strzok, who was removed from Mueller's team last year over the texts.
It's not yet clear if Comey would appear voluntarily. After he was fired by Trump, the three congressional committees investigating Russia all sought Comey's testimony, but he chose only to appear before the Senate Intelligence Committee. The committees would have the ability to subpoena Comey to compel his testimony.
A lawyer for Comey said he hasn't heard from either committee.
Comey has also appeared on Capitol Hill already to talk about the email investigation when he testified in September 2016 before the House Oversight Committee, which was then led by former Rep. Jason Chaffetz of Utah.
Since then, new details have emerged, such as Strzok's text messages and his role in editing Comey's public statement on Clinton.
One person that Republicans aren't anticipating on speaking with at this point: Clinton herself.
"At this point, this investigation is more about the process, not the verdict," Meadows said. "What we're concerned about is the process that took place."
Gaetz said there were "a lot of other facts we're looking to develop before we would get to that question" of whether Clinton would need to testify.
In addition to Comey, Gaetz said the list of witnesses is also likely to include other FBI officials who took part in drafting Comey's statement clearing Clinton of criminal wrongdoing but criticizing her handling of the private email server.
Comey's statement called Clinton's handling of the server to "extremely careless," but that was changed from "grossly negligent" in an earlier draft.
Strzok, who led the investigation of Clinton's private email server as the No. 2 official in the counterintelligence division, was responsible for the change, CNN has reported.