SAN FRANCISCO (AP) – Mayor Gavin Newsom took his first public step toward seeking higher office Tuesday, filing papers to form an exploratory committee for a potential gubernatorial bid in 2010.

Newsom, a Democrat best known for challenging California’s ban on same-sex marriage in 2004, is six months into his second and final term, which expires in 2011. Speculation over whether he would run to succeed Gov. Arnold Schwarzenegger has pursued him since before his re-election, but the mayor always deflected it by saying his attention remained fixed on the city.

The establishment of an exploratory committee will allow Newsom to start raising money and conducting polls. In announcing the committee, Newsom’s campaign consultant, Eric Jaye, said it demonstrated the mayor’s custom of “listening to people before acting” and that would let him launch a campaign “from a position of knowledge and strength.”

“He is looking forward to having a dialogue with Californians about their future in the coming few months,” Jaye said in a statement.

If he ends up entering the race, one of the 40-year-old Newsom’s opponents for the Democratic nomination could be a man 30 years his senior: Attorney General Jerry Brown, who previously served as governor from 1975 to 1983, has said he’s contemplating trying to reclaim the job.

Brown, 70, hasn’t established an exploratory committee devoted to the governor’s race, but has changed the name of his campaign committee from Jerry Brown for Attorney General to Jerry Brown 2010, which reported taking in more than $200,000 in donations during the last two weeks.

Newsom has $250,000 left over from his mayoral campaign, but the new gubernatorial committee won’t have to file its first campaign disclosure report until the end of the year, according to Jaye.

Newsom first made headlines early in his first term when he directed city workers to grant marriage licenses to gay couples, three months before Massachusetts became the first U.S. state to legalize same-sex marriage.

Some Democrats, including California’s senior U.S. senator, Dianne Feinstein, blamed him for helping fuel a conservative backlash that propelled President Bush to re-election that same year, while some political observers speculated that he may have irrevocably doomed his chance to secure a statewide office.

With the passage of time, though, his position has looked less revolutionary. Ruling in a lawsuit the city helped bring, the California Supreme Court, which four years earlier had nullified the gay marriages performed in San Francisco, struck down the state’s bans on same-sex nuptials in May.

Besides his reputation as a same-sex marriage advocate, Newsom has won praise and imitators in other cities for programs to get homeless people into housing combined with social services, promoting the use of tap over bottled water and providing employer-subsidized health care for uninsured residents.

Meanwhile, a trio of former Silicon Valley millionaires are eyeing the Republican nomination to succeed Schwarzenegger, with Insurance Commissioner Steve Poizner at the head of the pack.

The former technology firm head and presidential intern signaled his interest in the governorship early, pumping $2.5 million of his own money into a campaign against a November term limits initiative that would have allowed a handful of powerful Democrats to stay on in Sacramento.

Since then, he’s crisscrossed the state holding public events on insurance rate reductions for consumers and touting crackdowns on insurance scofflaws. He’s also appeared with the celebrity governor at disaster zones in the wake of wildfires.

Former Hewlett-Packard Co. CEO Carly Fiorina, who is feverishly working the California fundraising circuit on behalf of presumed GOP presidential nominee John McCain, has also been mentioned as a possible candidate. Another name floated for the state’s top job is ex-eBay Inc. president Meg Whitman, who became a national co-chairwoman of the McCain campaign after serving as Mitt Romney’s finance chairwoman.

Dan Schnur, a Republican media strategist who is not affiliated with any potential candidates, said both Brown, with his strong name reconition, and Newsom, with his solid liberal credentials, would be strong primary contenders.

“The conventional wisdom is that the same-sex marriage issue is a double-edged sword for Newsom, but I’d argue that it makes him the most likely Democratic nominee,” Schnur said. “An issue like this in the Democratic primary gives him an ability to break out of the pack.”