Gabbard is being accused by former Secretary of State Hillary Clinton of being a Russian asset. According to Clinton, the Kremlin is "grooming" Gabbard to run as a third-party candidate, to spoil the 2020 presidential election in order to help President Donald Trump.
Clinton made the remarks in a recent interview, without offering evidence to support her claim.
"Tulsi is not being groomed by anyone," O'Rourke dismissed Clinton's accusations.
"She is her own person," he continued.
The Texas Democrat pointed out that Gabbard is a combat veteran, who still serves in the United States military, reminding reporters that she is also a sitting congresswoman and a presidential candidate.
"Obviously 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."
Entrepreneur Andrew Yang, who is also running for president, dismissed the accusations as well, arguing that other politicians need to show more respect toward someone who is a combat veteran.
Independent Congressman Justin Amash went a step further in his claims, suggesting that Clinton herself is actually an assent -- not a Kremlin asset, but a Trump asset.
"The thing we know for sure is that Hillary Clinton is a Donald Trump asset," he said, arguing that Clinton's attacks are counterproductive, and only pushing independent-minded voters toward Trump.
Amash also suggested that similar accusations only help Trump, de-legitimizing real concerns about his connections with Moscow.
"Recognize that this plays right into Trump's hands; that it diminishes the legitimate inquiry into Russia; that it bolsters Trump's 'hoax' nonsense," Amash said.
Popular CNN hosts Erin Burnett and Van Jones criticized Clinton as well.
Burnett accused the former secretary of state of pushing her own "Russia conspiracy theory," and Jones implored Clinton and other prominent leaders to refrain from amplifying unsubstantiated claims.
Jones said that Gabbard is being attacked "with a complete smear and no facts."