"I think everybody agrees that in one way or another, it impacted the election," the California Democrat said on "The Situation Room with Wolf Blitzer" Wednesday. "Should (Comey) have done it? Probably not. But knowing him, it was on the spur of the moment and he just did it."
The comments came hours after Comey's hearing, during which the official strongly defended his choice to send a letter to Congress on October 28, 2016 that suggested the FBI was looking into Clinton's email server again after new emails had been discovered on a computer shared by Anthony Weiner and top Clinton aide Huma Abedin.
"It was a hard choice, I still believe in retrospect the right choice," Comey told senators at a judiciary committee hearing on oversight of his agency. "I can't consider for a second whose political fortunes will be affected."
Feinstein, the top Democrat on the Senate Judiciary Committee, said she "generally" has confidence in Comey.
"I think overall he is a straight shooter," she said. "He always has been with me. I think he's very -- he's not influenced by much other than his purpose and his job. And I think that's good."
But in this case, she said,"something happened that I don't know about."
"'Mildly nauseous' doesn't sound like he was too nauseated by it," Feinstein said. "But I think it was bad thing to do, and really for me a very disappointing thing for him to do."
She said Comey alerting Congress of the probe 11 days before election was an "October surprise."
"One of the things he said to us, well, one of the reasons for ... holding the Clinton press conference 11 days before the election, was there was such great interest in this," Feinstein said. "Well, let me tell you, when Russia hacks into our election systems, there's exceptional interest in that too. So that argument doesn't hold up. But he is so sure that he did the right thing and maybe I would say it wasn't the right thing if there was new information."
Feinstein said Comey could have instead gotten a search warrant to look through the computer and "find there was nothing new there."
Democrats -- including Clinton herself -- have pointed to Comey's probe as what cost them the White House.
"I was on the way to winning until a combination of Jim Comey's letter on October 28 and Russian WikiLeaks raised doubts in the minds of people who were inclined to vote for me and got scared off," the 2016 Democratic presidential nominee said Tuesday in an interview conducted by CNN's Christiane Amanpour at the Women for Women International summit in New York.
Sen. Cory Booker, a New Jersey Democrat, also called Comey actions "unacceptable.
"I wish he would come clean and say he didn't have to do this [Clinton announcement]," he said Wednesday on CNN's "Erin Burnett OutFront." "It's outrageous he is defending himself like this ... this is a man not taking responsibility."
President Donald Trump criticized Comey on Tuesday on Twitter.
"FBI Director Comey was the best thing that ever happened to Hillary Clinton in that he gave her a free pass for many bad deeds!" he tweeted. "The phony......Trump/Russia story was an excuse used by the Democrats as justification for losing the election. Perhaps Trump just ran a great campaign?"