The Federal Bureau of Investigation is looking into the incident, one of the officials said.

“This attempt is further proof that there are constant threats as we head into midterm elections, and we must remain vigilant in order to prevent future attacks,” said Bob Lord, chief security officer for the Democratic National Committee. He called the voter file “the party’s most sensitive information.”

The combination of this attack on the committee, continued influence operations by Russia and others using social media, and efforts to breach think tanks underscores that the cyber age has changed elections forever. So many systems are vulnerable to manipulation, from the voter-registration systems in the 50 states to the inner workings of the parties, that the opportunities for foreign and domestic manipulation are many.

“The focus has gone from using the cyber realm to steal secrets to using it to impose national power,” Rob Joyce, the White House cybersecurity coordinator until the position was eliminated a few months ago, said this month at Defcon, an annual convention that focuses on hacking and defenses against it.

Ahead of the midterms, political groups and government organizations have been on high alert for new hacks and foreign meddling. Microsoft said this week that it had detected that hackers tied to Russian intelligence targeted the Senate and conservative think tanks in the United States, also by creating fake websites.

And after Facebook last month identified a political influence campaign aimed at potentially disrupting the midterms, the company said this week that it had uncovered other disinformation efforts targeting people around the world.