Harris, the first woman and the first minority to serve as California's top prosecutor, planned to announce the move Tuesday morning, according to an adviser not authorized to discuss her plans who spoke to The Associated Press on condition of anonymity.
Harris, a former two-term San Francisco district attorney, is a friend of President Obama's and attracted national attention when she helped negotiate a settlement with major mortgage lenders and secured extra funding for California. She has been widely viewed as an eventual candidate for governor or U.S. senator.
The disclosure of her plans came just a few hours after a potential rival, California Lt. Gov. and former San Francisco Mayor Gavin Newsom, said he would not run for the open seat created by Boxer's retirement next year.
Former Los Angeles Mayor Antonio Villaraigosa and Tom Steyer, a retired San Francisco hedge fund billionaire who sought to make climate change an issue in the midterm elections, are also considering bids for the seat that Boxer has held for over two decades. Democrats are well-positioned to retain the seat in a state where the party controls every statewide office and both chambers in the Legislature.
As the state's chief law enforcement officer, Harris has focused her crime-fighting efforts on cross-border gangs that she says are increasingly engaged in high-tech crimes such as digital piracy and computer hacking to target businesses and financial institutions.
Harris, the daughter of an Indian mother and black father, was elected California attorney general in 2010.
Newsom's exit provided encouragement for others contemplating a Senate run, and his statement did nothing to dampen the idea he would run for governor in 2018 — when Gov. Jerry Brown's term ends.
"I know that my head and my heart, my young family's future, and our unfinished work all remain firmly in the state of California — not Washington, D.C. Therefore I will not seek election to the U.S. Senate in 2016," said Newsom, who has three young children.
Newsom launched a brief campaign for governor before dropping out in 2009. He is best known for ordering the San Francisco city clerk in 2004 to ignore state law at the time and issue marriage licenses to same-sex couples.
Freking reported from Washington, D.C.
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