US Senator Elizabeth Warren of MA on 31 December announced she had formed an exploratory committee for a White House run in what is expected to be a crowded Democratic primary field before the November 2020 presidential election.
According to CNN, Senator Sanders's 2016 deputy campaign manager Rania Batrice will be Ms Gabbard's campaign manager.
Gabbard, 37, said "the issue of war and peace" would be the main focus of her campaign.
"There are many reasons I'm offering to serve you as President - to ensure every American gets the healthcare they need, to bring about comprehensive immigration reform, to make sure we have clean water and clean air for generations to come, to fix our broken criminal justice system, to end the corrupt influence of special interests in Washington, and so much more".
Gabbard is so far left that she lost the support of a Hawaii teacher's union over her handling of Syria.
If elected, the 37-year-old lawmaker would be the youngest president in USA history.
Ms Gabbard was one of the most prominent lawmakers to back Sanders over Hillary Clinton in the 2016 Democratic presidential primary.
The Iraq war veteran has become the first-ever Hindu to be running for the presidency in the US. She could find herself competing against the likes of such heavy hitters as Sen. A source close to Gabbard confirmed to CBS News that she is running.
She questioned whether Assad was responsible for a chemical attack on civilians that killed dozens and led the U.S. to attack a Syrian air base. Though Gabbard has been aligned with Trump over some aspects of Middle East policy ― including whether the US should provide support for Syrian militias fighting Assad ― she recently called Trump "Saudi Arabia's bitch" for his failure to react to the Saudi killing of journalist Jamal Khashoggi.
California senator Kamala Harris, New Jersey senator Cory Booker and Vermont senator Bernie Sanders are all weighing their own presidential bids and are expected to announce decisions in the upcoming weeks.
In particular, she was criticized for meeting with Syrian president Bashar Al-Assad in 2017.
In response, Gabbard wrote on Twitter that "being Saudi Arabia's bitch is not 'America First'". She said "whoever is found responsible" must be held accountable and talked about "if" Assad is indeed guilty.
Gabbard was deployed to Iraq in 2005, and she maintains her role as a member of the Hawaii National Guard.
Former Vice President Joe Biden, who comes from the party's centrist wing, is also likely to join the fray.