Lamenting that the American Dream and its democracy are under attack in Donald Trump's United States, California Senator Kamala Harris formally declared Sunday her candidacy to become the country's first black female president in 2020.

Harris, who was California's attorney general from 2011 to 2017, made her announcement to supporters outside city hall in Oakland, San Francisco's grittier neighbor where she was born.

Barack Obama introduced himself to Californian voters in 2007 in the same location before he was elected the first African American president of the United States a year later.

Harris already announced in a January 21 video that she would run for president. She joins a Democratic field with several other candidates seeking to keep Trump out of the White House for a second term.

"We are here because the American dream and our American democracy are under attack and on the line like never before," she said.