“I’m running for president of the United States, and I’m very excited about it,” Harris, 54, told ABC’s “Good Morning America.”
Harris paired the announcement with the release of a campaign video on Twitter in which she said, "Truth. Justice. Decency. Equality. Freedom. Democracy. These aren’t just words. They’re the values we as Americans cherish. And they’re all on the line now."
"The future of our country depends on you and millions of others lifting our voices to fight for our American values," Harris said. "That’s why I’m running for president of the United States."
The former California attorney general was elected to the Senate in 2016. Since then, she has worked to establish a national profile -- by aggressively questioning President Trump’s judicial nominees, writing a book and stumping for Democrats in last year's midterm elections.
Her announcement comes as some Democrats, emboldened with their new majority in the House, have suggested impeaching the president. Asked on ABC on Monday if she believes Trump has committed an impeachable offense, Harris wouldn’t say, but said it’s important that Special Counsel Robert Mueller continue his investigation into whether the Trump campaign colluded with the Russians in the 2016 race.
She plans a formal campaign launch in Oakland on Jan. 27. The campaign will be based in Baltimore, with a second office in Oakland.
Harris, who is black, launched her presidential as the nation observes what would have been the 90th birthday of the slain civil rights leader Martin Luther King Jr.
Still, her record as a prosecutor and state attorney general has attracted new scrutiny from liberals as she has inched closer to a presidential run.
University of San Francisco associate law professor Lara Bazelon recently argued in an op-ed piece that the perception that Harris acted as a “progressive prosecutor” during her tenure as the district attorney of San Francisco and then California’s attorney general contradict her actions.
"Time after time, when progressives urged her to embrace criminal justice reforms as a district attorney and then the state’s attorney general, Ms. Harris opposed them or stayed silent,” Lara Bazelon wrote in the New York Times.
But Republicans are taking her seriously as a top-tier candidate.
“At 54, Harris is two decades younger than some of her septuagenarian competitors – an age that enables her to appeal to the Instagram crowd without being painted as inexperienced,” said Colin Reed, a Republican strategist who worked for former Republican Sen. Scott Brown. “A child of immigrants, she brings diversity to a party obsessed with racial and gender politics."
Harris’ announcement comes as a slew of Democrats have begun making plans to run for the White House in 2020.
In recent weeks, New York Sen. Kirsten Gillibrand, Massachusetts Sen. Elizabeth Warren, Hawaii Rep. Tulsi Gabbard and former San Antonio Mayor Julian Castro have moved forward with plans to seek the party’s nomination.
Other prominent figures, including former Vice President Joe Biden, Vermont Sen. Bernie Sanders and former Texas Rep. Beto O’Rourke, are also mulling possible campaigns.
Fox News' Louis Casiano and Jennifer Girdon and The Associated Press contributed to this report.