So far, there is no evidence that the Russians have changed any vote tallies or voter registration information, officials said. They added that the Russian-backed hackers had penetrated the computer networks without taking further action, as they did in 2016.

But American officials expect that if the presidential race is not called on election night, Russian groups could use their knowledge of the local computer systems to deface websites, release nonpublic information or take similar steps that could sow chaos and doubts about the integrity of the results, according to officials briefed on the intelligence. Such steps could fuel Mr. Trump’s unsubstantiated claims that the vote is “rigged” and that he can be defeated only if his opponents cheat.

Some U.S. intelligence officials view Russia’s intentions as more significant than the announcement on Wednesday night by the director of national intelligence, John Ratcliffe, that Iran has been involved in the spread of faked, threatening emails, which were made to appear as if they came from the Proud Boys, a right-wing extremist group.