Federal officials battling foreign interference in the coming election say they are entering what may be the most perilous period of the campaign: October, when the prospect for mischief runs high and the time to react runs short.

While the Russians make little attempt to hide their support for former President Donald J. Trump, the Iranians — among Moscow’s most important suppliers in the war in Ukraine — desperately want to stop him from returning to office, and are busy hacking into his campaign and dumping whatever they find, as well as plotting to assassinate him. The Justice Department is expected to announce, likely as soon as Friday, the indictment of an Iranian hacking group closely associated with the Islamic Revolutionary Guard Corps for the hack of the Trump campaign, along with efforts to attack the accounts of Washington journalists and others.

And China, once expected to be a major player in the election, seems uncertain which candidate it detests more. So, for now, Beijing is focusing on local races, conducting influence operations that have the potential to undermine public faith in the basic democratic process.

That has been the essence of a series of private intelligence briefings for election workers and members of Congress recently, as U.S. officials describe the stark, oftentimes confusing battlefield in which disinformation ramps up and the risk of cyberattacks is greatest.

At first glance, those briefings sound right out of the playbooks from 2016 and 2020. During one such meeting on Sept. 13, U.S. officials said Russia remained “the most active foreign influence threat” and other countries were flooding the internet with messages meant to “stoke discord and undermine confidence in our electoral process.”

But as the briefing went on it was clear that what may unfold in the next month and a half includes some new features that American election officials have never seen before.