“This is the first big test of the 2020 race: Is the press, are Democrats, are Democratic activists, going to let that happen again?” said Jennifer Palmieri, who was the Clinton campaign’s director of communications.

She and other former Clinton aides have been watching with an apprehensive sense of deja vu as they see Trump solicit a foreign government’s help in attacking a political adversary.

“You need to be aggressive so people know what’s true and what’s not,” she said. “You can’t pick your fights. . . . It’s a terrible situation for a campaign to be in. But that’s life when you’re dealing with Donald Trump.”…

Zac Petkanas, the director of rapid response for Clinton’s 2016 campaign, said Democrats may have miscalculated four years ago in ignoring conspiracy theories that sprouted on the fringes of the Internet. They quickly went mainstream, Clinton’s campaign realized.

“The false attacks really were effective at sowing doubt, not only within a right-wing sphere, which is what we originally thought, but within our own supporters,” Petkanas said.