Washington (CNN) Democratic presidential candidate Sen. Bernie Sanders on Monday defended his fellow 2020 hopeful Rep. Tulsi Gabbard of Hawaii against the claim that she is a "Russian asset," saying the suggestion is "outrageous."

"Tulsi Gabbard has put her life on the line to defend this country," the Vermont independent tweeted . "People can disagree on issues, but it is outrageous for anyone to suggest that Tulsi is a foreign asset."

Sanders does not name Hillary Clinton in the tweet, but his comments appear to be aimed at the 2016 Democratic presidential nominee, whose claim last week on a podcast with former Obama adviser David Plouffe that the Russians are "grooming" a Democrat in the 2020 presidential primary to run as a third-party candidate and champion their interests has caused a backlash. Clinton also accused 2016 Green Party presidential candidate Jill Stein of being a "Russian asset."

Clinton's team on Monday declined to comment to CNN about Sanders' tweet.

Gabbard, who endorsed Sanders over Clinton during the 2016 presidential election, responded to her on Friday, calling her "queen of warmongers, embodiment of corruption, and personification of the rot that has sickened the Democratic Party." Gabbard also said in a tweeted video message on Sunday, "If they can falsely portray me as a traitor, then they can do it to anyone."

"That's exactly the message that they want to get across to you: that if you stand up against Hillary and the party power brokers, if you stand up to the rich and powerful elite and the war machine, they will destroy you and discredit your message," Gabbard said.

"I am not a Russian spy," Stein said. "I think this is a completely unhinged conspiracy theory for which there is absolutely no basis in fact. Not for myself and not for Tulsi Gabbard. I think it's really outrageous that Hillary Clinton is trying to promote this crazy idea."

Clinton's team, after receiving bipartisan criticism, said the former secretary of state was saying that Republicans and President Donald Trump were grooming the congresswoman.

"Folks, listen to the podcast. She doesn't say the Russians are grooming anyone. It was a question about Republicans," Clinton spokesperson Nick Merrill tweeted on Friday.

Sanders is not the only 2020 Democratic candidate to weigh in on the controversy.

Former Rep. Beto O'Rourke of Texas also defended Gabbard, saying she "is not being groomed by anyone" and that the focus of the campaign should be on the economy, climate change and other issues affecting Americans.

"That's not correct. Tulsi is not being groomed by anyone. She is her own person," he told reporters after delivering a keynote address Saturday at the Alabama Democratic Conference Semi-Annual Convention in Birmingham. "Obviously (she) has served this country, continues to serve this country, in uniform, in Congress, as a candidate for presidency, so I think those facts speak for themselves."

South Bend, Indiana, Mayor Pete Buttigieg dismissed the claim on Sunday, saying that "there is no basis for that" and "statements like that ought to be backed by evidence."

"I don't know what the basis is for that," he told CNN's Jake Tapper on "State of The Union." "But I consider her to be a competitor. I respect her service. I also have very different views than she does, especially on foreign policy, and I would prefer to have that argument in terms of policy, which is what we do at debates and what we're doing as we go forward."

Sen. Elizabeth Warren of Massachusetts said she didn't have any comment when asked about the controversy in Des Moines, Iowa, on Monday.