The hearing is sure to be explosive. Last month, President Trump suddenly fired Comey amid an FBI investigation into whether the Trump campaign coordinated with the Kremlin to influence the 2016 election. While the Trump administration said the firing was over Comey's handling of the Hillary Clinton email investigation, the president has said the Russia probe was on his mind when he removed his FBI director — who is appointed to a 10-year term to avoid political influence.
Comey will almost certainly not be able to discuss details of the ongoing investigation, which is now being led by a special counsel. He might, though, be able to talk about the details of his firing and other conversations he had with the president.
Comey wrote in private notes that Trump asked him to shut down the FBI's probe into former national security adviser Michael Flynn, and he also kept notes on a conversation in which he alleged Trump asked him for an affirmation of loyalty, people familiar with the documents have said.
Comey has reached an understanding with the special counsel's office about what he can and cannot make public, people familiar with the matter have said.
Comey has a history of turning congressional testimony into must-watch TV: in 2007, he famously described to lawmakers of how he intervened in the hospital room of then-Attorney General John Ashcroft to stop, at least for a while, the approval of a National Security Agency surveillance program that he believed to be unlawful.
His appearance comes just as the special counsel's probe into Russian meddling in the election is heating up. In recent weeks and months, a grand jury has issued subpoenas to request records related to Flynn's businesses, and investigators are now scrutinizing Jared Kushner, the president's son-in-law and a top White House adviser, for meetings he had with Russians in December.
Separately on Thursday, Sens. Patrick J. Leahy (D-Vt.) and Al Franken (D-Minn.) released letters they had sent to Comey asking him to investigate whether then Attorney General-nominee Jeff Sessions broke the law in telling Congress, wrongly, that he had not had communications with Russians during the campaign. In fact, Sessions had spoken twice to Russia's ambassador to the United States.
The senators said that since Comey's firing they had "been in communication with the FBI concerning a response to our letter," and they expected to be briefed soon.
Karoun Demirjian contributed to this report.