Gabbard, 37, has represented Honolulu's suburbs and rural Hawaii in the US House since 2013. She is a combat veteran who served in Iraq and Kuwait with the Hawaii National Guard.
She was an early, vocal supporter of Vermont Senator Bernie Sanders during the last presidential campaign, which has made her popular with progressive Democrats.
Gabbard indicated in an interview with CNN earlier this month that she would be a candidate. She said she would follow it up with a formal launch, which occurred Thursday.
Critics have focused on her past advocacy against same-sex marriage. In one example, she spoke at a news conference in 2004 when she was a state representative about federal judges "tearing apart our US Constitution in order to force same-sex marriage down the throats of the people of Hawaii and America".
She appeared on behalf of Alliance for Traditional Marriage and Values, an organisation founded by her father to lobby against same-sex marriage.
Lee Cataluna, a columnist for the Honolulu Star-Advertiser, wrote last week, "the unedited truth is that she didn't just slip and say a bad word, she actively, passionately worked against the legalization of same-sex marriage. She belonged to a group of rabid advocates of 'traditional' marriage."
To address such criticisms, Gabbard released a nearly four-minute-long video statement on YouTube last week, explaining how she grew up in a socially conservative household but has since formed her own opinions shaped by her life experiences. She apologised for her past statements and the harm they caused.
"I look forward to being able to share more of my story and experiences growing up -- not as an excuse, but in the hopes that it may inspire others to truly live aloha; to love and care for others," she said.
Amid the blowback, Democratic state Senator Kai Kahele announced last weekend he would run for her seat in Congress. Gabbard hasn't indicated whether she would run for re-election as she also seeks the presidency.
Gabbard has said she's motivated by US foreign policy that has led the country into military conflict in Iraq, Libya and Syria.
She said the actions have in the Middle East have destabilised the region, made the US less safe and cost thousands of American lives. At the same time, terrorist groups like Al Qaeda and the Islamic State group are stronger than before the September 11 terrorist attacks, she said.
In her new video Thursday, she said all Americans have paid the price for what she called "interventionist, regime-change wars".
"We have spent trillions of your taxpayer dollars to pay for these wars, taking those dollars away from our communities and our people who need them right here at home," she said.
In 2017, she travelled to Syria to meet with Syrian President Bashar Assad. She came under intense criticism for the trip, including from some Democrats, because of accusations Assad's government is guilty of war crimes and even genocide during the country's civil war.
Gabbard also got heat from Democrats for meeting Donald Trump to talk about Syria when he was president-elect.
Gabbard stands out from other candidates in many ways. She's the first Hindu to be elected to Congress and is the first born in American Samoa. She's also an avid surfer.
She's among the youngest, along with Pete Buttigieg, the 37-year-old mayor of South Bend, Indiana, seeking the presidency.
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