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Tom Steyer, the billionaire environmental activist who spent over $70 million on the 2014 midterms, announced Thursday that he will not pursue a run to replace retiring California Democrat Sen. Barbara Boxer.

In a blog on The Huffington Post, Steyer wrote that the decision "about whether to engage from the outside or seek elected office came down to a single question: How best can I fight for a level playing field at this point?"

"Given the imperative of electing a Democratic president -- along with my passion for our state -- I believe my work right now should not be in our nation's capital but here at home in California, and in states around the country where we can make a difference," he said.

California Attorney General Kamala Harris has announced a run for the seat, and former Los Angeles mayor Antonio Villaraigosa is also said to be considering a bid.

- Carrie Dann