Comey thrust Sessions back into the spotlight of the roiling Russian Federation controversy with his incendiary appearance before the Senate Intelligence Committee last week - the same panel that will interrogate the attorney general.
That remark came after revelations emerged that Sessions had met with Russia's ambassador to the US last year, despite testifying under oath during a confirmation hearing that he "did not have communications with the Russians".
Warren emphatically believes that Sessions' involvement in Comey's dismissal would be a violation of his recusal.
Comey also has said Sessions did not respond when he complained he didn't "want to get time alone with the president again".
It has never been clear how Sessions could have recused himself from the Russian Federation investigation, but still involved himself in the decision to fire the FBI director, given that the decision to fire Comey was directly linked to the Russian Federation investigation.
Senator Patrick Leahy, the Senate appropriations committee's top Democrat and a member of the Senate judiciary committee, tartly reminded Sessions that both oversee his department.
If Sessions chooses to opt out of answering certain questions, legal experts say the committee can hold him in contempt - but that could elongate the testimony and could potentially take months.
Diane Marie Amann, a law professor at University of Georgia, agreed with Spicer that invoking privilege was possible: "It depends on the questions that are asked", she said.
There is a way that Sessions could skirt around having to directly answer these questions, however. Did you ask your deputy attorney general to write a memo recommending the firing of Comey?
In the private, classified meeting that followed, Comey reportedly told senators of yet another, undisclosed meeting between Sessions and Russian government officials - bringing the total known number of Sessions' covert contacts with the Russian government to three, so far.
As president-elect, Trump nominated the Republican Alabama senator in November to serve as his attorney general. The Justice Department has denied that, saying Sessions stressed to Comey the need to be careful about following appropriate policies. Comey declined to elaborate in an open setting and Sessions accepted the intelligence committee's invitation to appear in part so he could address those comments.
Despite the report, White House principal deputy press secretary Sarah Huckabee Sanders said Thursday in response to a question from a reporter that Trump "absolutely" has confidence in Sessions.
While Sessions' excuse for the ditching one hearing for the other checks out, there could be another reason for the abrupt change of plans. Sessions wants a chance to respond - questions about whether the FBI was in possession of some facts about Sessions in Russian Federation that would make it clear Sessions would have to recuse from that investigation.
Maryland Attorney General Brian Frosh joined District of Columbia counterpart Karl Racine at a news conference in announcing a lawsuit against Trump, filed in U.S. District Court in neighboring Maryland.
Aides initially told reporters that the hearing would likely be closed, reportedly raising concerns among some committee members that Sessions was trying to avoid testifying publicly. "Totally illegal - very cowardly".