Working with Mr. Trump’s Justice Department, Mr. Graham has released investigative records raising doubts about the integrity of the agents running the case and information upon which they relied, though Democrats have said they lack context and are misleading. The president, in turn, has eagerly taken the disclosures as ammunition in a long-running campaign to reshape the public’s understanding of the inquiry, which concluded the Trump campaign willingly accepted help from the Russians and that Mr. Trump sought to foil the investigation.
The committee has already publicly questioned two former deputy attorneys general, Rod J. Rosenstein and Sally Yates, who oversaw aspects of Crossfire Hurricane and signed off on the applications for the secret wiretap warrants targeting Mr. Page. Both expressed regret for errors identified by Mr. Horowitz but dismissed assertions by Republicans on the panel that their actions were politically motivated or that Mr. Trump’s campaign need not have been investigated.
Democrats have opposed Mr. Graham at every turn, accusing him of abusing his Senate powers to help Mr. Trump and take attention from the continuing Russian threat. On Wednesday, they said he was unfairly trying to discredit the entire investigation based on one small aspect of it.
“Those errors were serious, but the errors and the so-called Steele dossier — and this is important — played no part in the broader Russia investigation,” said Senator Dianne Feinstein of California, the panel’s top Democrat. She noted that of the 10 people interviewed in the committee’s investigation, not one had claimed anything different.
Mr. Comey is offering a forceful defense.
Mr. Comey, who had not testified before Congress since Mr. Horowitz’s report was released in December, remained steadfast in his decision to open the investigation, arguing that the F.B.I. had sufficient reason to scrutinize the ties between the Trump campaign and Russia.
“In the main, it was done by the book, it was appropriate and it was essential that it be done,” Mr. Comey said under questioning by Mr. Graham. “Overall I am proud of the work, but there are parts of it we will talk about that are concerning.”
Mr. Comey has previously described the problems with the wiretap as “real sloppiness.”
“But remember how we got here. The F.B.I. was accused of criminal misconduct,” Mr. Comey said in December on “Fox News Sunday.” “Remember, I was going to jail, and lots of other people were going to jail. People on this network said it over, and over, and over again.”