Los Angeles (CNN) Hours after California Attorney General Kamala Harris began raising money for a bid to replace Barbara Boxer in the U.S. Senate, billionaire environmentalist Tom Steyer outlined the case for his own run Tuesday and said he would decide soon -- setting up the likelihood of an exorbitantly expensive contest that could have two Democrats facing off in November of 2016.

California's two Senate seats have been locked down by Boxer and U.S. Sen. Dianne Feinstein since 1992, closing off statewide opportunities for the next generation of politicians . Under California's new top-two primary system, the top two candidates in the first round will advance to the general election, regardless of their party.

In a column on the Huffington Post website, Steyer said he was intrigued by the opportunity of taking on interests in Washington that "oppose recognizing global warming and fight against the rights and futures of average Americans."

"Washington needs to be shaken up and we need climate champions who will fight for the next generation," Steyer said. "California Democrats are blessed to have a deep bench of talent, and I will decide soon based on what I think is the best way to continue the hard work we have already started together to prevent climate disaster and preserve American prosperity."

Steyer and Harris would be among the most formidable contenders for Boxer's seat, along with former Los Angeles Mayor Antonio Villaraigosa, who has said he is "seriously considering" a run. Several members of the California congressional delegation and of the state leadership have signaled interest, including Rep. Loretta Sanchez, Rep. John Garamendi, and state Treasurer John Chiang.

In a Facebook post on Tuesday, Sanchez touted her focus on working families, immigration reform and affordable healthcare: " I have advocated for policies that give all families the same access to the American Dream," she said. "Californians deserve a strong voice in Washington and I have never been afraid to speak up, which is why I am seriously considering running for the United States Senate in 2016."

But lesser-known candidates could face great difficulty raising the money needed to run. Boxer raised some $35 million in conjunction with party committees for her 2010 race against former Hewlett-Packard chief executive Carly Fiorina.

Steyer, a former hedge fund manager from San Francisco whose net worth has been estimated at $1.6 billion by Forbes, has pledged to give away the bulk of his fortune to public interest and philanthropic causes during his lifetime. He has been a vocal opponent of the Keystone XL pipeline and a free-spending advocate for California environmental causes.

His NextGen Climate Action Committee spent more than $74 million in the midterm elections -- much of it Steyer's money -- trying to defeat candidates who had expressed doubt about climate change, but they had little demonstrable success in a Republican wave year.

As a candidate, Steyer's investments throughout his business career are also likely to come under uncomfortable scrutiny. Last year, for example, the New York Times examined lucrative investments by Steyer's former fund, Farallon Capital Management in companies across the globe that own coal-fired power plants

Harris, the former district attorney of San Francisco who was just re-elected to her second term as attorney general, was the first to formally declare her candidacy since Boxer announced last week that she would not run for re-election in 2016.

While Villaraigosa could build a strong Southern California base, and draw on his support from California's growing Latino community, Harris also has a solid fundraising base in California from her two statewide runs and her close alliance with President Barack Obama.

She has won praise as attorney general for pursuing banks that did not follow proper procedures when they foreclosed on the properties of California homeowners and she has kept a steady focus on reducing recidivism and truancy.

"From my first days as a prosecutor in Alameda County, to my work as San Francisco District Attorney to my current service as California Attorney General, I have worked to bring smart, innovative and effective approaches to fighting crime, fighting for consumers and fighting for equal rights for all," Harris said in a statement on her new campaign web page Tuesday.

"I will be a fighter for the next generation on the critical issues facing our country. I will be a fighter for middle class families who are feeling the pinch of stagnant wages and diminishing opportunity," she continued. "I will be a fighter for our children who deserve a world-class education, and for students burdened by predatory lenders and skyrocketing tuition. And I will fight relentlessly to protect our coast, our immigrant communities and our seniors."

Harris' position was strengthened on Monday when Lt. Gov. Gavin Newsom bowed out of contention . Speculation had swirled around the possibility of a heated race between Harris, 50, and Newsom, 47, who could have potentially split the Northern California vote, clearing the way for a Southern California candidate like Villaraigosa.

Newsom has told many confidantes that he is interested in running for governor when Jerry Brown faces term limits in 2018, and he and Harris share the same political consultants. In a statement on his Facebook page, Newsom said it was better "to be candid than be coy" and noted that it would be better for his three children, Montana, Hunter and Brooklynn, if he stayed in California.

"While I am humbled by the widespread encouragement of so many and hold in the highest esteem those who serve us in federal office, 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," Newsom said.

Hinting at his potential alliance with Harris, he added that in the months to come, he looked "forward to doing whatever I can to help elect California's next great Democratic senator -- one worthy of succeeding Barbara Boxer and serving this remarkable state of dreamers and doers in the United States Senate."

Shortly after Boxer announced that she would retire from the Senate in 2016, Los Angeles Mayor Eric Garcetti said he would not run for the seat.