Harris' bid, announced online, sets up the prosecutor as a strong contender in a rapidly growing field of potential candidates.

"I will be a fighter for the next generation on the critical issues facing our country," Harris said in the announcement. "I will be a fighter for middle-class families who are feeling the pinch of stagnant wages and diminishing opportunity."

Boxer's departure, announced last week, is the first of three anticipated retirements among California's top leaders that should clear the way for a younger generation of politicians who have been waiting in the wings.

Governor Jerry Brown, 76, will leave office at the end of his fourth term in 2018 because of term limits. U.S. Senator Dianne Feinstein, 81, will be 85 when her term ends.

California Lieutenant Governor Gavin Newsom, whose support for same-sex marriage while mayor of San Francisco spurred a wave of social and political change, said on Monday he would not run.

His announcement leaves the field open to potential candidates including Harris, 50, and former Los Angeles Mayor Antonio Villaraigosa, 61, both Democrats.

Harris, like Newsom, rose from the San Francisco political scene and is a former district attorney who has used her position to develop policy ideas as well as handle prosecutions.