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 Wednesday.

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

Sponsors of the ban declared victory early Wednesday, 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 92 percent of precincts reporting early Wednesday, the ban had 5,010,855 votes, or 52 percent, to 4,650,469 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.

Similar bans had prevailed in 27 states before Tuesday's elections, but none were in California's situation — with about 18,000 gay couples married since a state Supreme Court ruling in May.

Spending for and against the amendment reached $74 million, making it the most expensive social-issues campaign in U.S. history and the most expensive campaign this year outside the race for the White House.

Some in San Francisco vowed to continue fighting for the right to marry if the proposition does pass. "My view of America is different today," said Diallo Grant, a gay man with mixed-race parents. "The culture wars will continue."