The retiring Nevada Democrat said Democrats "would have won the majority in the Senate and would have won the presidency but for Comey."
"It's obvious he was a partisan in all this," Reid told CNN's Manu Raju in an interview. "There's information out there. He had it, I'm confident. And he ignored it."
He said Comey "significantly" helped make Trump the President-elect, faulting his failure to condemn Russian hacking of Democratic operatives and his handling of the investigation into Democratic presidential nominee Hillary Clinton's email server.
Reid said he urged Comey to address the hacking, which intelligence agencies have concluded was the result of Russian efforts to tilt the election toward Trump.
"I am saying the FBI did nothing -- did nothing. All the information that we've heard in the last couple weeks, it was available to the FBI. He just ignored it. He did not make it public. We asked him to more than once and he didn't do it," Reid said.
He also pointed to Comey's letter to lawmakers less than two weeks from the election announcing that -- as part of the FBI's investigation into sexting allegations against Anthony Weiner, the husband of Clinton aide Huma Abedin -- it had discovered emails potentially relevant to its investigation into Clinton's handling of classified information on her private email server.
"In fact, to show how awful this situation is, this man ignored precedent that had been going on for decade after decade after decade. The FBI does not get involved in politics -- except Comey did," Reid said.
"Had he not written that letter a week or so before the election, she would have won; we would have picked up at least two more Senate seats," he said.
Reid's comments came less than two hours after Senate Majority Leader Mitch McConnell, R-Kentucky, said he supports a Senate investigation into Russian hacking.
"I think we ought to approach all of these issues on the assumption the Russians do not wish us well," McConnell said.
Trump has brushed off the CIA's conclusion that Russia hacked the Democratic National Committee and Clinton campaign chairman John Podesta in a bid to help the Republican nominee.
The President-elect on Sunday morning blasted the Intelligence Community anew, calling its assessment that Russia interfered in the election "ridiculous."
"I think it's just another excuse. I don't believe it," Trump said in an interview on "Fox News Sunday."