“Any attempt to interfere with American elections must be treated as a threat to our vital national security interests,” Mr. McCain said after questioning Adm. Michael S. Rogers, the director of the National Security Agency, at the hearing. “Yet so far, the only response from the Obama administration has been a warning from the secretary of defense.”

It is unclear if Mr. Obama raised the issue with President Vladimir V. Putin of Russia when they met 10 days ago in China. If an F.B.I. investigation finds overwhelming evidence of Russian government responsibility in the hacking of the Democratic National Committee or other recent breaches, the president would have to decide whether or how to respond.

Any response may not be made public, since Mr. Putin is seen as responding better to private warnings, or demonstrations of retaliation, than to anything that openly challenges his authority.

One reason officials like Mr. Comey have expressed confidence in the security of the vote is that most states (Pennsylvania is a notable exception) keep a paper backup of votes as they are cast. Paper trails are not foolproof. But their existence means that after a lengthy audit — even one as politically paralyzing as the 2000 recount in Florida — an accurate count would probably be made.

Voting machines are not supposed to be connected to the internet (though there are some exceptions), providing some additional measure of protection. But results are reported online, and one fear that federal cyberexperts have discussed is that a sophisticated “man in the middle” attack could allow hackers to take over internet systems used to report unofficial results on election night. Such a breach might not alter the official ballot count, but it could sow deep mistrust about the numbers that are broadcast.

“If I were a bad guy, I know I could manipulate the system,” said Stuart McClure, the author of “Hacking Exposed” and the chief executive of Cylance, a company that uses artificial intelligence techniques to repel attacks on networks. “But the problem here is not really about manipulation. It’s about the damage you can do with disruption, so that people lose confidence in the system.”