Her announcement kicked off a day of tribute and outreach to black voters by Democratic presidential candidates and potential candidates. Senators Cory Booker of New Jersey and Bernie Sanders of Vermont made appearances, as did Ms. Gillibrand, Ms. Warren and former Vice President Joseph R. Biden Jr., who expressed regret for supporting crime bills earlier in his career that have been criticized for disproportionately impacting black Americans.

Ms. Harris’s long-expected entry comes as many Democrats are eager to find new leaders and as the party grasps for a unifying message that can appeal to its increasingly progressive base and more moderate voters who have recoiled from President Trump.

A 54-year-old former California attorney general and San Francisco district attorney, Ms. Harris is something of a bridge between the Democrats eyeing the race who are in their 70s, like Mr. Biden and Mr. Sanders, and those in their 40s, like Mr. Booker and former Representative Beto O’Rourke. Further, while she hails from one of the country’s most famously liberal cities, she has ties to both the pragmatic and leftist wings of the party: She is rooted in the Bay Area’s Democratic establishment but has embraced a more progressive agenda since being elected to the Senate in 2016.

While other Democrats have focused their economic policy on regulating corporations or limiting the influence of Wall Street, Ms. Harris’s signature proposal is more focused on individuals and would provide lower-income families with monthly cash payments of up to $500. She has also proposed a bail reform bill that’s backed by several civil rights groups, and she focused her initial Senate work on curbing maternal death rates, particularly among black women.

Like many Democrats, Ms. Harris has sought to align herself with the party’s leftward drift in recent years, proclaiming her support for “Medicare for All” and, after an initial hesitation, disavowing most corporate donations. She has also embraced the legalization of recreational marijuana, a position she once rebuffed.

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“She has long been a reform-minded prosecutor who struck a balance between the need for public safety and reckoning with civil rights ideals,” said Kristen Clarke, president and executive director of the Lawyers’ Committee for Civil Rights Under Law.

But not everyone agrees that Ms. Harris can easily assuage Democrats worried about her record as a prosecutor and district attorney.