McCabe is scheduled to testify Wednesday before the Senate Intelligence Committee, along with Deputy Attorney General Rod Rosenstein, Director of National Intelligence Dan Coats, and National Security Director Adm. Mike Rogers, all one day prior to Comey's hearing before the same senators.

Republican senators on the committee reportedly are poised to question McCabe as to why neither he nor Comey informed the Justice Department about President Trump's efforts to shut down the Russia investigation. On Thursday, they plan to grill Comey on the matter as well. Republicans have indicated they will argue if Trump actually did anything potentially outside the law — yet Comey never informed anyone at the Justice Department about it — that would raise serious questions about Comey's credibility as a potential witness against Trump.

It is unclear whether McCabe will answer questions on his private conversations with Comey about whether to tell the Justice Department about Trump's alleged pressure of the former FBI director. The new special counsel might believe that such information is pertinent to his investigation, and it is possible that Mueller has asked McCabe not to discuss the issue.

Mueller, himself a former FBI director, may have a bias in believing the word of Comey and other FBI managers over that of Trump. Comey and Mueller became professionally close, and also personal friends, while Comey was the deputy attorney general for part of the Bush administration and Mueller was FBI director. Comey succeeded Mueller in the job.

When he appointed Comey, then-President Barack Obama cited a 2007 confrontation that Comey had with senior officials of the Bush administration. President George W. Bush's then–chief of staff, Andrew Card, and then–White House counsel, Alberto Gonzales, sought a reauthorization of the administration's warrantless wiretapping program by then-Attorney General John Ashcroft, who was in the hospital at the time, barely coherent, and recovering from surgery.

Ashcroft, Comey, and Mueller believed that portions of the program were illegal. Comey and Mueller raced to Ashcroft's hospital room to warn him not to sign the authorization. All three men considered resigning in protest if the program were reauthorized. Bush agreed to modify the program to bring it into compliance with the law.

During the incident, Card summoned Comey to meet him at the White House. In a harbinger of current events, Comey only agreed to go if he could bring a witness, then–Solicitor General Ted Olson.

When the controversy became publicly known, Card and Gonzales claimed that Comey and Mueller's accounts were untrue. Yet when the Justice Department's inspector general investigated the matter, Card and several other Bush administration officials declined to speak to investigators at all.

When the inspector general interviewed Comey and Mueller, both men served as witnesses for each other, corroborating in part one another's accounts. Investigators concluded their accounts were truthful and accurate.

This was largely because both men kept detailed, contemporaneous, and at times even legible notes on the incident. It was also because both men gave investigators the names of subordinates — as many as a half-dozen for each of them — who were themselves involved in the events, or who were told about them by Comey and Mueller shortly after the fact.

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