Graham confirmed Comey told Senate and House select committees on intelligence about the email but he never indicated it was fake.
"He talked to members of the Senate and House intel committee that he was sitting on emails that the Russians had been between the Democratic Party and the Department of Justice that were highly explosive," he said on CNN's State of the Union on Sunday (28 May).
"He never once told a member of the House or the Senate that he thought the email was fake," Graham added.
Graham, who is not a member of the Senate intelligence committee, is leading a Senate judiciary subcommittee probe into Russian interference in the 2016 election.
The Washington Post reported on Wednesday (24 May) that the FBI obtained a document detailing an alleged email from then-Democratic National Committee (DNC) chief Debbie Wasserman Schultz.
The alleged email said someone at the Department of Justice told Wasserman Schultz that Attorney General Loretta Lynch was containing the probe into Hillary Clinton's emails.
The Post's report said the details came from "a dubious Russian document," which influenced Comey to make his view on the investigation public. CNN later reported that Comey acted on the document despite knowing it was fake.
"I can't imagine a scenario where it's OK for the FBI director to jump in the middle of an election based on a fake email generated by the Russians and not tell the Congress," Graham said.
The senator said that if the email described in the Russian document was fake, the US needs to ramp up sanctions against Russia. However, Graham did not dismiss the possibility that the email was real. "I want to get to the bottom of it," he said. "I want to see the email."
Graham also expressed doubts on reports that President Trump's son-in-law Jared Kushner attempted to set up a secret communications channel between the Trump transition and Moscow.
"Number one, we are chasing our tails as a nation when it comes to the Russians," the Republican senator said. "I don't know who leaked this information, but just think about it this way—you've got the Ambassador of Russian reporting back to Moscow on an open channel, 'Hey, Jared Kushner's going to move into the embassy!'"
Graham added: "I don't trust this story as far as I can throw it!"