US Senator Kamala Harris on Monday announced that she is running for president, joining an already-crowded field of Democrats who are making or considering bids to take on President Donald Trump.

"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 senator representing California said in a video posted on Twitter.

After two terms as district attorney of San Francisco (2004-2011), she was twice elected as attorney general of California (2011-2017), becoming not only the first woman but the first black person to serve as chief law enforcement officer of that populous state.

Then in January 2017, she took the oath of office as California's junior US senator, making her the first woman of South Asian descent (her mother is a Tamil Indian) and only the second black woman senator in American history, after Carol Moseley Braun.

Her focused and often tough-sounding lines of questioning during closely watched Senate hearings reflect her past as a prosecutor.

Harris often proudly recalls that as a prosecutor she fought big banks during the 2008 financial crisis to defend families.

She casts herself as a champion of middle class families "living paycheck to paycheck" and denounces police brutality and the killing of unarmed black men.

Nearly 22 months before the 2020 election, the battle for the White House is already firming up, as Americans begin to assess who might be the opposition party nominee to challenge Trump for control of the White House.