Most of that voter registration is public. But Ratcliffe said that government officials "have already seen Iran sending spoofed emails designed to intimidate voters, incite social unrest and damage President (Donald) Trump."
Ratcliffe was referring to emails sent Wednesday and designed to look like they came from the pro-Trump Proud Boys group, according to government sources.
U.S. intelligence agencies previously said that Iran might interfere to hurt Trump and that Russia was trying to help him in the election.
Outside experts said that if Ratcliffe was correct, Iran would be trying to make Trump look bad by calling attention to support and threats by the sometimes violent group.
Iran summoned the Swiss envoy on Thursday to protest against what it called "baseless" U.S. claims.
"Iran's strong rejection of American officials' repetitive, baseless and false claims was conveyed to the Swiss ambassador," Foreign Ministry spokesman Saeed Khatibzadeh told state TV. "As we have said before, it makes no difference for Iran who wins the U.S. election."
Switzerland represents U.S. interests in Iran because Washington and Tehran have no diplomatic ties. Tensions have risen between the longtime foes since 2018, when Trump exited Iran's 2015 nuclear deal and stepped up sanctions on Tehran.
The Kremlin also denied the allegation.
"Such accusations appear every day, they are unfounded and not based on anything," said Kremlin spokesman Dmitry Peskov.
U.S. National Security Adviser Robert O'Brien told reporters at the White House on Thursday that the attempted election interference appears to have affected two to three U.S. cities and counties and involved a "small amount of information."
U.S. Senate Democratic leader Chuck Schumer, who received a classified briefing on Wednesday afternoon on election security, said he disagreed with Ratcliffe that Iran was specifically trying to hurt Trump.
"It was clear to me that the intent of Iran in this case and Russia in many more cases is to basically undermine confidence in our elections. This action I do not believe was aimed ... at discrediting President Trump," Schumer told MSNBC.
White House spokesman Judd Deere said Trump has directed government agencies "to proactively monitor and thwart any attempts to interfere in U.S. elections, and because of the great work of our law enforcement agencies we have stopped an attempt by America’s adversaries to undermine our elections."
The emails are under investigation, and one intelligence source said it was still unclear who was behind them.
Another government source said that U.S. officials are investigating whether people in Iran had hacked into a Proud Boys network or website to distribute threatening materials. This source said U.S. officials suspect the Iranian government was involved but that the evidence remains inconclusive.
Some of those emails also contained a video, debunked by experts, that purported to show how fake ballots could be submitted. Ratcliffe said that claim was false.
The second government source said U.S. authorities have evidence that Russia and Iran had tried to hack into voter roll data in unidentified states. But the source added that because much of that voter data is available commercially, the hacking may have been aimed at avoiding payment.
(Reporting by Sarah N. Lynch, Joseph Menn, Mark Hosenball, Lisa Lambert, Jason Lange, Michelle Nichols, Raphael Satter, and Alexander Marrow Writing by Stephen Nellis Editing by Kim Coghill, Michael Perry and Frances Kerry) ((firstname.lastname@example.org; +44 207 542 6136; Reuters Messaging: email@example.com))