Billionaire climate activist Tom Steyer announced Thursday that he will not enter the race to succeed U.S. Sen. Barbara Boxer in California, a move that reshaped the early contest that has attracted a pack of potential candidates.

Steyer, 57, was seriously considering a 2016 bid after Boxer said this month she would step down at the end of her term.

The wealth of Steyer, a Democrat and former hedge fund manager, and his involvement in environmental issues that resonate in left-leaning California could have made him a strong contender.

He said in a statement on The Huffington Post that global warming will "define the success or failure of our generation," and the nation needs leadership in, and outside of, government.

"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 wrote.

State Attorney General Kamala Harris -- another Democrat -- has entered the race and at least a dozen others are considering bids or are widely viewed as potential contenders, including former Los Angeles Mayor Antonio Villaraigosa.

"Tom Steyer has shown himself to be a real champion for change in his passionate campaign for a cleaner and healthier world," Villaraigosa said in a statement.

For Harris, the announcement by Steyer erased a prominent potential rival.

"Harris is a very formidable candidate," Jack Pitney, a political science professor at Claremont McKenna College, said before Steyer's decision. "The only candidate who could keep Kamala Harris awake at night would be Tom Steyer, only because of the amount of money he can spend."

Democrats are well-positioned to retain the Senate seat held by Boxer in the state where the party controls every statewide office and both chambers in the Legislature.

Tom Del Beccaro and Duf Sundheim, two former chairmen of the California Republican Party, are also considering runs for the office.

Steyer will be seen as a prospect for a future campaign and signaled he intends to remain active in politics. Seeking to make climate change an issue, he poured about $74 million into 2014 races.

"The road we take may be less traveled and less well marked, but I am very determined. The journey is far from over -- in fact, it has just begun," he wrote.