The development leaves Attorney General Jerry Brown, who has formed an exploratory committee, as the only Democrat making a serious bid to succeed Gov. Arnold Schwarzenegger next year.

A statement issued by Newsom's campaign said he was unable to devote the time needed to run an effective campaign, citing "a young family and responsibilities at City Hall."

His announcement was not a surprise to those following the Democratic campaign. Newsom, 42, was unable to find the same popularity throughout California that he enjoys in his home city.

Brown had a 7-to-1 fundraising edge over Newsom and heavy financial support from unions, a core Democratic constituency, even though Brown has not officially announced his candidacy. A Field Poll earlier this month showed Newsom trailing Brown by 20 points among likely Democratic voters.

Newsom failed to gain traction even after holding months of town hall meetings throughout the state and ringing up an endorsement from former President Bill Clinton.

A Clinton fundraiser that had tickets selling at up $50,000 a piece failed to give Newsom the financial boost he needed to compete with a likely run by Brown.

Newsom's exit clears the way for the 71-year-old Brown, who was elected to the first of his two terms as governor in 1974.

Sen. Dianne Feinstein would be the favorite if she chose to enter the Democratic field, a decision she has said she would not make until early next year.

The Field Poll showed neither Brown nor Newsom came close to having Feinstein's appeal. Gil Duran, a spokesman for the senator, said "no immediate comment" when asked for her reaction to Newsom's withdrawal from the race.

Newsom's campaign hinged on his appeal as a progressive, as he attempted to tap into the currents of change that helped sweep Barack Obama into the White House last year.

His successes in San Francisco on such issues as providing universal health care and promoting alternative energy became pillars of his gubernatorial campaign.

Newsom is in his second term as mayor of California's fourth-largest city and is best known for directing city agencies in 2004 to issue marriage licenses to same-sex couples. While the courts quickly stopped the practice, Newsom's actions sent the debate over gay marriage to the forefront of California politics, beginning a series of court challenges and ballot initiatives.

Newsom struggled to define himself beyond gay marriage in the rest of the state. In last year's successful ballot campaign to ban same-sex marriage in California, supporters presented Newsom as the face of gay marriage advocates.

Newsom also entered the governor's race on the heels of an unseemly affair with the wife of his mayoral campaign manager. After admitting the dalliance in 2007, he sought treatment for a drinking problem.

After winning re-election later that year, he seemed to catch a second wind in office. Last year, he married actress Jennifer Siebel, who gave birth to the couple's first child, a daughter, last month.

In bowing out of the race, Newsom said he will continue to pursue the issues for which he cares deeply _ health care, a cleaner environment, education and equal rights.

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Williams reported from Sacramento. Staff Writer Michael R. Blood in Los Angeles also contributed to this report.