US Warns Russia: Don't Mess With Our Election

— -- The U.S. government has warned Russia not to interfere with the presidential election today or face "serious consequences," a senior U.S. official told ABC News.

The private warning followed an unusual public condemnation from America's top intelligence official, Director of National Intelligence James Clapper, who said the Kremlin's "senior-most officials" must have authorized cyberattacks on American individuals and political institutions, including the hack of the Democratic National Committee this summer.

"The recent disclosures of alleged hacked emails... are consistent with the methods and motivations of Russian-directed efforts," the Office of the Director of National Intelligence said in a joint statement with the Department of Homeland Security on Oct. 7. "These thefts and disclosures are intended to interfere with the U.S. election process."

In addition to the hacking of emails related to the 2016 presidential campaign, which have continued up to Election Day, the U.S. intelligence community said in October that some U.S. states have "recently seen scanning and probing of their election-related systems, which in most cases originated from servers operated by a Russian company," though the U.S. said it was not in a position to blame the Russian government for that. Russian officials have denied the hacking allegations.

Already at least four U.S. states have had some part of the security surrounding their voter registration databases breached, which is separate from the voting systems, U.S. officials previously told ABC News, and in response 46 states took the DHS up on its offer to provide cyber "hygiene" services to root out any vulnerabilities in their systems.

Cybersecurity researchers and experts have long warned that the U.S. election process is vulnerable to cyberattacks, even down to hacking individual voting booths. But while U.S. officials have acknowledged the threat, they've remained publicly confident that hackers would not be able to actually impact the election's outcome. They said the system is so complex and antiquated, and often backed up by paper ballots, that large-scale disruption is virtually impossible.

"The [U.S. Intelligence Community] and the Department of Homeland Security (DHS) assess that it would be extremely difficult for someone, including a nation-state actor, to alter actual ballot counts or election results by cyber attack or intrusion," the DHS and ODNI said in their statement. "This assessment is based on the decentralized nature of our election system in this country and the number of protections state and local election officials have in place."

Some experts, however, have pointed out that big moments in nationwide elections have come down to particular key districts in key states -- meaning a targeted attempt to throw off the vote there could have an out-sized impact.

"A slight alteration of the vote in some swing precincts in swing states might not raise suspicion," former White House cybersecurity advisor and current ABC News consultant Richard Clarke wrote in a news analysis titled "Yes, It's Possible to Hack The Election" in August. "I have to emphasize that we have no evidence that such hacking has ever taken place in the U.S. or that it is about to occur. What we do know is that it could happen."

U.S. officials also told ABC News a major concern involves a cyberattack that wouldn't even have to be a successful hack of the system - just one big enough to undermine Americans' confidence in the system or to sow confusion about the results.

ABC News' Lee Ferran contributed to this report.