Democratic senator Kamala Harris, who represents California, says she will run for president in the 2020 election.
Senator Harris, who was elected in 2016, previously served as the state’s attorney general.
“I love my country,” she told ABC’s Good Morning America, adding she would “fight for the best of who we are”.
A vocal critic of President Donald Trump, she is the fifth Democrat to join the battle for the party’s nomination in the 2020 election.
Elizabeth Warren, Kirsten Gillibrand, Tulsi Gabbard and John Delaney have also announced they are intending to run.
“The future of our country depends on you and millions of others lifting our voices to fight for our American values. That’s why I’m running for president of the United States,” the 54-year-old senator said in a video posted on Twitter.
The 2020 Democratic presidential primary will be the first time more than one woman competes for the party’s nomination.
Near the head of the pack
Analysis by Anthony Zurcher, BBC News, Washington
Kamala Harris is the kind of Democrat who could stick around and prevail in what is sure to be a gruelling nomination battle. She is from California, which is rich in both primary delegates and fundraising dollars. As a woman, and from an ethnic minority, she is well positioned to capitalise on her party’s growing diversity.
She has one of the most liberal voting records in the US Senate at a time when Democrats are leaning to the left, but she also has a background as a hard-nosed prosecutor.
That background may end up a vulnerability as well, given that some progressives are already criticising her prosecutorial record. She will have to walk a fine line to tout her accomplishments while justifying her decisions.
Sen Harris has only been on the national stage two years, and not every political neophyte can hold up under fire the way Mr Obama did in 2008. Sen Harris will be tested in the coming months, but she starts the contest near the head of the pack.
Securing the nomination would make Sen Harris the first African American to be a major party nominee for the presidency.
“I have the unique experience of having been a leader in local government, state government and federal government,” she said. “The American public wants a fighter… and I’m prepared to do that.”
Who is Kamala Harris?
She served two terms as district attorney of San Francisco (2004-2011), before being elected as attorney general of California (2011-2017), the first woman of colour to do so.
In 2017, the former prosecutor was sworn in as California’s junior US senator.
She is the daughter of immigrants from Jamaica and India and has pushed back against critics of “identity politics”.
“It is used to try and shut us up,” Sen Harris told a conference last summer.
Her tough questioning of Justice Brett Kavanaugh about his views on abortion and the ongoing investigation into alleged Russian interference in the 2016 election attracted attention from Democrats.
But she has also faced criticism for saying she was not aware of allegations of sexual harassment against one of her aides, who resigned in 2016.
Sen Harris plans to launch her campaign during a rally in Oakland, California, on Sunday.