Democratic presidential contender Tulsi Gabbard sued Hillary Clinton for defamation on Wednesday, seeking at least $50 million in damages for harming her reputation by suggesting last year that one of the party's White House contenders was a "Russian asset."

Tulsi Gabbard
The Gabbard lawsuit said Clinton had refused to apologise or retract the comments, which it described as "devastating to a United States politician's reputation," and made them knowing they were false.

The lawsuit said Clinton's comments had damaged the presidential candidacy of Gabbard, a US representative from Hawaii, and were motivated by anger over Gabbard's endorsement in 2016 of Clinton's Democratic nominating contest rival, Bernie Sanders.

"Clinton got exactly what she wanted by lying about Tulsi - she harmed her political and personal rival's reputation and ongoing presidential campaign, and started a damaging whisper campaign based on baseless, but vicious, untruths," said the lawsuit, filed in federal court in New York.

"That's ridiculous," Clinton spokesman Nick Merrill said of the lawsuit.

Clinton, the Democratic presidential nominee in 2016, had said on a podcast in October that Russians were grooming one of the Democratic presidential candidates for a third-party bid, but did not name the candidate.

"She's the favourite of the Russians, they have a bunch of sites and bots and other ways of supporting her so far, and that's assuming Jill Stein will give it up, which she might not, because she's also a Russian asset," Clinton said.

Stein was the Green Party candidate in the 2016 presidential election.

Clinton referred to Gabbard

After the podcast comments, Merrill confirmed to Politico that Clinton was referring to Gabbard.

The Gabbard lawsuit said Clinton had refused to apologise or retract the comments, which it described as "devastating to a United States politician's reputation," and made them knowing they were false.

"Rep. Gabbard must defend her good name and hold Clinton responsible. This lawsuit intends to do just that," Gabbard's lawyer, Brian Dunne, said in a statement.

As a public figure, Gabbard must show that the defendant acted with actual malice to recover damages. In addition, US courts have said "rhetorical hyperbole" associated with politics is constitutionally protected speech.

Gabbard has routinely polled in the law single digits in the Democratic race to pick a challenger to Republican President Donald Trump in November's election, but she has said she will not run as a third-party candidate.

After Clinton's comments in October, Gabbard said the former first lady, US senator and secretary of state was the "embodiment of corruption and personification of the rot that has sickened the Democratic Party for so long."