Mr. Comey said he had “no idea what on earth” Mr. Barr meant by saying the F.B.I. lacked sufficient reason to begin investigating the Trump campaign. He accused Mr. Barr of acting “at odds with the nature of the department” to remain outside the political fray.

“When the attorney general starts acting like the personal lawyer for the president, it threatens that, and that is a priceless thing,” Mr. Comey said.

The former director also took on the most direct claim related to the investigation that Republicans have leveled against former Vice President Joseph R. Biden Jr., the Democratic nominee for president.

Mr. Comey denounced an unfounded assertion pushed by Republicans — including Mr. Trump at the first presidential debate on Tuesday — that in an Oval Office meeting in the final days of the Obama administration, Mr. Biden came up with the legal theory for prosecuting Mr. Trump’s former national security adviser Michael T. Flynn, who twice pleaded guilty to lying to the F.B.I. about his contacts with the Russian ambassador.

The Justice Department has released handwritten F.B.I. notes that Mr. Trump and his allies claim as supporting evidence. But former F.B.I. officials say Republicans are misrepresenting the notes and taking them out of context.

Mr. Comey, who attended the meeting, said that Mr. Biden had played no role in directing the investigation. “I would remember it because it’d be highly inappropriate for a president or vice president to suggest prosecution or investigation of anyone — and it did not happen,” Mr. Comey said.

Mr. Graham spent much of the hearing building his case around the unverified intelligence made public by Mr. Ratcliffe, in an apparent bid to help Mr. Trump politically. The years-old intelligence, rejected by other investigators, suggested that Russian intelligence officers had acquired information that Hillary Clinton had approved a plan for her 2016 campaign to elevate concerns about Mr. Trump’s ties to Russia.

Mr. Comey said he was not aware of the intelligence. “I have read Mr. Ratcliffe’s letter, which frankly I have trouble understanding,” he said.