Same-sex marriage advocates in Florida, Arizona, and California may face their toughest opponents in the wave of new, minority voters set to hit the polls next month.

Initiatives on those states’ Nov. 4 ballots would define marriage as being between one man and one woman, thereby prohibiting same-sex marriage.

Gay-rights supporters worry not only that Democratic presidential hopeful Barack Obama has pledged to oppose such efforts but also that his candidacy will bring out record numbers of blacks and Hispanics to vote for him. Those two groups vote mostly for Democratic candidates but often are cultural and religious conservatives who tend to oppose same-sex unions.

About 6 percent of California voters are black, and 15 percent are Hispanic.

“It’s a Catch-22,” Andrea Shorter, campaign director of California’s gay and civil rights coalition And Marriage for All, told The New York Times.

Obama’s tepid support for same-sex marriage compounds the issue for gay-rights activists. Obama has said he favors “extending fully equal rights and benefits to same-sex couples under both state and federal law,” according to a letter he wrote in June in a letter to the San Francisco-based Alice B. Toklas LGBT [Lesbian Gay Bisexual Transgender] Democratic Club.

His letter described California initiative Proposition 8 as a “divisive and discriminative” effort to amend the Golden State’s Constitution.

But that sanction for civil unions is a long way from an endorsement for same-sex marriage, a political hot potato that neither Obama nor Republican rival Sen. John McCain seems eager to juggle into a major campaign issue.

Obama has indicated that, if elected president, he would change the military’s “don’t ask, don’t tell” policy of keeping sexual orientation private, a compromise that Democratic President Bill Clinton put in place in 1993. Obama also supports repealing the Defense of Marriage Act, which Clinton signed into law in 1996.

Such a repeal could require all states to accept same-sex marriages from Connecticut, California and Massachusetts, the three states where such unions are legal.

Obama used to say that each state should be able to make its own laws regarding same-sex marriage, but his former love of states’ rights apparently has changed.

And few doubt that Obama would appoint liberal judges inclined to legalize same-sex marriage from the bench, as California Supreme Court justices did in May by overturning an earlier marriage-defining initiative that 61 percent of voters made state law.

Ironically, liberal Obama thus could become the pivotal factor that passes anti-same-sex-marriage initiatives in three states.

Such unintended interactions between ballot propositions and candidates have happened before, but never before in precisely this way.

In 2004, a same-sex marriage ban initiative on the ballot “brought out enough conservative voters in Ohio to swing the state to GOP President Bush, giving him the state’s 20 electoral votes and a national victory,” according to San Francisco Chronicle reporter John Wildermuth.

Pundits have discussed for more than a year whether Obama might fall victim to the so-called “Bradley Effect,” in which people trying to avoid appearing bigoted tell pollsters they will vote for a black candidate but do not. It was named for Los Angeles Mayor Tom Bradley, who narrowly lost his race to become California’s governor in 1982 after a near-election Field Poll predicted he would win by 5 percent.

Field Poll analysts concluded that factors other than race may have defeated Bradley. One such element was the fact that the ballot included a statewide initiative, which liberal Democrats such as Bradley pushed, that would have restricted the legal possession of handguns.

This gun control initiative “brought out a skewed electorate different from the model used to predict likely voters,” Field Poll director Mark DiCamillo reportedly told veteran San Francisco Bay Area Democratic operative Frank D. Russo.

In other words, it brought an unexpectedly large number of Second Amendment conservatives to the polls to vote down the initiative. They also voted for Bradley’s Republican opponent, George Deukmejian, who became governor.

After the election next month, political scientists and pollsters might coin a new term, the “Obama Effect,” if his name on the ballot helps pass anti-same-sex marriage initiatives.

As of late September, California’s Field Poll showed Proposition 8 trailing by 55 percent to 38 percent.

“There’s no question African-American and Latino voters are among our strongest supporters,” said Frank Schubert, co-manager of the Yes on 8 campaign behind the measure. “And to the extent that they are motivated to get to the polls, whether by this issue or by Barack Obama, it helps us.”

By early October, Yes on 8 had begun buying heavy airtime for its first TV ad. The ad features San Francisco’s controversial liberal Mayor Gavin Newsome, who defied state law in 2004 by performing illegal same-sex marriages (3,995 of which were later annulled by the California Supreme Court) at city hall, boasting: “This door’s wide open now. It’s wide open, whether you like it or not.”

The Yes on 8 ad then warns that, unless outlawed, same-sex marriage could be taught as normal behavior to young children in public schools and be forced into churches and other traditional social institutions via anti-discrimination laws and lawsuits.

The Proposition 8 pro and anti groups plan to spend up to $20 million each on this campaign.

The Knights of Columbus, a Catholic fraternal service organization, has committed $1 million to pass Proposition 8. The American Family Association has committed $500,000 to do likewise, and the lobbying arm of Focus on the Family has spent $450,000.

On the other side, Bruce Bastian, co-founder of WordPerfect software, reportedly donated $1 million to the opponents of Proposition 8.

And to further complicate this issue, University of Southern California constitutional law expert David B. Cruz has noted that Proposition 8 would ban same-sex marriage, but not the legal requirement that heterosexuals and homosexuals be treated equally under law. Therefore, Proposition 8 could create a legal situation in which “no one could get married in California,” Cruz said.

Four years ago, 11 state ballots carried similar same-sex marriage ban initiatives. All won, by margins of at least 60 percent in every state except Oregon and Michigan, without the “Obama Effect” to boost them. Traditional marriage continues unchanged in those states.

In Arizona, Proposition 102, the Marriage Protection Amendment, was placed on the ballot by 16 Republican state senators, including the Senate president, and by 30 members of Arizona House. Recent polling found it leading by 49 percent to 42 percent.

This measure, as well as those in Florida and California, are supported by Arizona McCain. But in 2004, McCain opposed a Republican-backed constitutional amendment to outlaw same-sex unions nationwide.

Arizona law presents same-sex marriage. Proposition 102 would make such prohibition a part of the Arizona Constitution to put it beyond the power of liberal judges, like those in California, to declare it unconstitutional, its supporters argue.

In 2006, Arizona voters rejected Proposition 107, which opponents argued would have prohibited not only same-sex marriage but also domestic partnerships and civil unions.

In Arizona Hispanics make up about 29 percent of the population and 17 percent of eligible voters. Four percent of the state population is African-American.

In Florida, Proposition 2 aims to put into the state Constitution the following language: “In as much as a marriage is the legal union of only one man and one woman as husband and wife, no other legal union that is treated as marriage or the substantial equivalent thereof shall be valid or recognized.”

Because this is an initiative to change the Florida Constitution, it must gain a 60 percent supermajority to win. Recent polling shows the measure with 55 percent support and 41 percent opposition.

In Florida, Hispanics total about 20 percent of the population and 14 percent of eligible voters. Fifteen percent of the state population is black.

“Especially among social conservatives, there have been a lot of things about McCain they didn’t like,” the head of Yes2Marriage, the group behind Florida’s Proposition 2, told the San Francisco Chronicle. “But this will give a lot of conservatives a reason to come to the polls.”

Obama is expected to win California easily, and Arizona native son McCain is expected to carry his home state by a solid margin.

“Florida is one state where same-sex marriage might have a serious effect,” said Jack Pitney, a political science professor at Claremont McKenna College. “It’s a close campaign there between McCain and Obama, and a point or two might make a difference.”

But if Obama wins in Florida, will his “Obama Effect” also put a hard-to-remove same-sex marriage ban into the Florida Constitution?

Obama’s transformative effect on American politics and culture may be bigger, and more surprising, than he ever anticipated.