Hillary Clinton has claimed a Democrat presidential candidate is being "groomed" by the Kremlin to run as an independent in 2020.
In an astonishing attack on Tulsi Gabbard, a congresswoman from Hawaii, Mrs Clinton suggested Russia would use her to damage the Democrats' chances of taking the White House.
Ms Gabbard, 38, responded by calling Mrs Clinton the "queen of warmongers" and the cause of "rot" in the Democrat party.
The bitter row began when Mrs Clinton was being interviewed about the prospect of Russian interference in the upcoming election.
She said: "I'm not making any predictions, but I think they've got their eye on somebody who's currently in the Democratic primary, and they're 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."
Mrs Clinton did not mention Ms Gabbard by name, but a spokesman later confirmed she had been referring to Ms Gabbard.
The spokesman said: "This is not some outlandish claim, this is reality."
Ms Gabbard is a military veteran who served in Iraq. She caused controversy after revealing that she had met with Bashar al-Assad on a fact-finding trip to Syria.
Responding to Mrs Clinton's allegations she said: "Thank you Hillary Clinton. You, the queen of warmongers, embodiment of corruption, and personification of the rot that has sickened the Democratic Party for so long, have finally come out from behind the curtain."
She accused Mrs Clinton of being behind a concerted campaign to derail her candidacy.
Ms Gabbard added: "It was always you, through your proxies and powerful allies in the corporate media and war machine, afraid of the threat I pose."
The congresswoman urged Mrs Clinton to run again in 2020. She said: "Don't cowardly hide behind your proxies. Join the race directly."
During the latest televised Democrat debate in Ohio this week Ms Gabbard condemned suggestions of Russian support for her.
She said: "This morning, a CNN commentator said on national television that I'm an asset of Russia. Completely despicable."
Mrs Clinton also accused Jill Stein, the Green Party presidential nominee in 2016, of being a "Russian asset".
In 2016 Ms Stein received about one per cent of the vote but some Democrats claim that helped Donald Trump win several key states.
Ms Stein denied Mrs Clinton's accusations and accused her of "peddling conspiracy theories to justify her failure, instead of reflecting on real reasons the Democrats lost in 2016."