“Malign foreign influence efforts in our elections have been a perennial problem,” Mr. Rosen said.
In recent weeks, William R. Evanina, a top intelligence official, has issued warnings about efforts by Russia, China and Iran to interfere in the election, including a stark admonition that Russia has spread disinformation about former Vice President Joseph R. Biden Jr. in an attempt to hurt his candidacy and lift Mr. Trump’s.
And top election security officials held a briefing with state election officials on Wednesday to update them on the latest threat reporting, including efforts by Russia, China and Iran to potentially influence election results.
The Department of Homeland Security and the F.B.I. have installed sensors on all state election computer networks as well as many local election systems to track any efforts by hackers to conduct reconnaissance or hack into voting systems, officials said.
The sensors have helped federal officials understand better the types of attempts to break into election systems. Most of the attempts have failed, according to homeland security officials, and largely amounted to efforts to find vulnerabilities that could be exploited later.
The Department of Homeland Security has been conducting tests and computer network security reviews, using information from those reviews and from the new sensors to develop warnings about vulnerabilities that hackers, including from foreign governments, could try to use before the election.
Some ransomware attacks on county governments have damaged election systems by locking them down, but those attacks were not aimed at halting voting, the federal officials said. They nonetheless prompted election officials to make adjustments.
Election security officials said they were worried about foreign influence operations not just before Election Day, but also in the days after, particularly if absentee voting creates delays in counting votes.
Federal officials will set up two operations centers to quickly address any problems, one for unclassified information and one for classified intelligence, that will begin functioning a week before the election and remain in place until local and state officials say the votes are counted. They will also set up a chat room for state and local officials to share information for a few days before and a few days after the election.