LOS ANGELES — A proposed ban on same-sex marriage in California — widely seen as the most momentous of the 153 ballot measures at stake nationwide — remained undecided early today.

The proposed constitutional amendment would limit marriage to heterosexual couples, the first time such a vote has taken place in a state where gay unions are legal.

Sponsors of the ban declared victory early today, but the measure’s opponents said too many votes remained uncounted for the race to be called.

Even without the wait, gay rights activists had a rough day Tuesday. Ban-gay-marriage amendments were approved in Arizona and Florida, and gay rights forces suffered a loss in Arkansas, where voters approved a measure banning unmarried couples from serving as adoptive or foster parents. Supporters made clear that gays and lesbians were their main target.

Elsewhere, voters in Colorado and South Dakota rejected measures that could have led to sweeping bans of abortion, and Washington became only the second state — after Oregon — to offer terminally ill people the option of physician-assisted suicide.

A first-of-its-kind measure in Colorado, which was defeated soundly, would have defined life as beginning at conception. Its opponents said the proposal could lead to the outlawing of some types of birth control as well as abortion.

In California, the night had started out optimistically for many who believed that a large Democrat-voter turnout would help defeat the state’s proposed ban on same-sex marriage.

With 90 percent of precincts reporting early today, the ban had 4,922,675 votes, or 52 percent, to 4,577,453 votes, or 48 percent, against.

Late absentee and provisional ballots meant as many as 3 million ballots were left to be counted after all precinct votes were tallied.

Elsewhere, the marijuana reform movement won two prized victories, with Massachusetts voters decriminalizing possession of small amounts of the drug and Michigan joining 12 other states in allowing use of pot for medical purposes.

Henceforth, people caught in Massachusetts with an ounce or less of pot will no longer face criminal penalties. Instead, they’ll forfeit the marijuana and pay a $100 civil fine.

The Michigan measure will allow severely ill patients to register with the state and legally buy, grow and use small amounts of marijuana to relieve pain, nausea, appetite loss and other symptoms.

Nebraska voters, meanwhile, approved a ban on race- and gender-based affirmative action, similar to measures previously approved in California, Michigan and Washington. Returns in Colorado on a similar measure were too close to call.

Ward Connerly, the California activist-businessman who has led the crusade against affirmative action, said Obama’s victory proved his point. “We have overcome the scourge of race,” Connerly said.

Energy measures met a mixed fate. In Missouri, voters approved a measure requiring the state’s three investor-owned electric utilities to get 15 percent of their electricity from renewable sources by 2021. But California voters defeated an even more ambitious measure that would have required the state’s utilities to generate half their electricity from windmills, solar systems, geothermal reserves and other renewable sources by 2025.

Two animal-welfare measures passed — a ban on dog racing in Massachusetts, and a proposition in California that outlaws cramped cages for egg-laying chickens.

Amid deep economic uncertainty, proposals to cut state income taxes were defeated decisively in North Dakota and Massachusetts.

In San Francisco, an eye-catching local measure — to bar arrests for prostitution — was soundly rejected. Police and political leaders said it would hamper the fight against sex trafficking. And in San Diego, voters decided to make permanent a ban on alcohol consumption on city beaches.

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Associated Press writer Paul Elias in San Francisco contributed to this report.