Mrs. Clinton made her allegation on “Campaign HQ,” a podcast hosted by David Plouffe, a former aide to President Barack Obama.

“I’m not making any predictions,” she said, “but I think they’ve got their eye on somebody who is currently in the Democratic primary and are grooming her to be the third-party candidate. She’s the favorite of the Russians. They have a bunch of sites and bots and other ways of supporting her so far.”

She did not name Ms. Gabbard in that interview, but her target was clear. Nick Merrill, a spokesman for Mrs. Clinton, told NBC News, “if the nesting doll fits,” when he was asked to confirm that Mrs. Clinton had been referring to Ms. Gabbard.

Disinformation experts have noted frequent mentions of Ms. Gabbard by RT, the Kremlin-backed news agency, and apparent Twitter bot activity amplifying pro-Gabbard messages and hashtags.