It is the newest, and most potent, of the litmus-test issues.
In 2000, gay marriage was a remote, abstract possibility to most Americans. Four years later, after a court decision legalizing marriage for gays and lesbians in Massachusetts, and with similar lawsuits working their way through the courts in other states, it has become one of the defining social issues of the 2004 presidential campaign.
It has galvanized liberals and conservatives, lending an intensity to their causes that was once the province of issues such as abortion. It has given shape and flammability to a deep divide over homosexuality, and has swollen the ranks on both sides of the chasm.
With proposed measures banning same-sex marriage on the ballots of 11 states in November, the controversy over gay marriage has helped mobilize political machinery and money across the nation, and has established a new kind of political minefield -- a fact made clear by the controversy over Senator John F. Kerry's mention of Vice President Dick Cheney's lesbian daughter in the final presidential debate last week.
Neither Kerry nor President Bush supports same-sex marriage. Both have said marriage should be reserved for the union of a man and woman.
In the debate Wednesday, Bush said he does not know whether homosexuality is a choice, and he urged tolerance for gays and lesbians. He has spoken in favor of a federal constitutional amendment banning gay marriage; Christian conservative groups who have made the ban their most important issue over the past year, applauded his words. (Congress did not advance the amendment).
Bush has spoken out against what he and conservatives call "activist" judges -- an apparent reference to judges like those on the Massachusetts Supreme Judicial Court, who found new rights to marriage in the Constitution.
"I think it's very important that we protect marriage as an institution between a man and a woman," Bush said in the debate. "I proposed a constitutional amendment. The reason I did so was because I was worried that activist judges are actually defining the definition of marriage, and the surest way to protect marriage between a man and woman is to amend the Constitution."
Although Kerry opposes gay marriage, he also is against a federal constitutional amendment banning it. He has said the matter should be left to the states, and has accused the president and other Republicans of playing politics with the Constitution and using the issue of gay marriage to divide the country.
Kerry has supported a constitutional amendment in Massachusetts banning gay marriage. During the debate, he said he thinks gays and lesbians like Mary Cheney -- who is openly gay -- do not choose their sexuality but are born with it. Kerry supports civil unions, which would confer some of the rights and responsibilities of marriage on gay and lesbian couples, and supports hate crimes legislation protecting them. He voted against the 1996 Defense of Marriage Act (which limits the benefits available to gay and lesbian couples, and which he says makes the Federal Marriage Amendment redundant) because he saw it as "gay-bashing on the floor of the US Senate."
In an interview in the latest issue of The Advocate, Kerry left himself some wiggle room on gay marriage.
"I have my view, and my view is my view," he said, when asked whether he would ever change his mind on the matter. "I can't tell you in 20 years or whenever, if someone made a persuasive argument, the world changes . . . So I don't predict the future. What I tell you is that my position is what it is."
Although Kerry has not embraced their signal cause, gay and lesbian activists have thrown themselves behind him, arguing that he is the most gay-friendly candidate ever to run for president.
Most surveys have indicated that Americans surveyed oppose gay marriage by overwhelming margins, but that they are less supportive of a federal constitutional amendment to ban it. A Pew Research Center poll in February found that 65 percent of respondents opposed gay marriage, but that only 22 percent of them favored an amendment. According to a Washington Post/ ABC News poll taken in the summer, 59 percent of respondents said it should be illegal for gay couples to marry, but only 44 percent favored a federal constitutional amendment. And when voters in many surveys are asked about the issues that most concern them, gay marriage consistently ranks near the bottom of the list: The Pew poll put it at 21 out of 22 items tested.
The legalization of gay marriage in Massachusetts, attempts to win marriage rights in other states, and the fractious ballot-question battles that have broken out all over the country have raised the issue's profile, particularly in states where the presidential race is close this year. Louisiana and Missouri have passed bans on gay marriage by overwhelming margins this year: In Missouri, 71 percent of the electorate voted for the ban.
"It's not too much of a surprise that several weeks after the Missouri referendum, the Kerry campaign pulled out of the state," said Karlyn Bowman, a poll analyst at the conservative American Enterprise Institute.
On Election Day, gay marriage bans will be on the ballot in such crucial battleground states as Ohio, Michigan, and Oregon.
Democrats and gay rights supporters see the ballot questions as cynical attempts by conservatives to use the issue of gay marriage to boost turnout for Bush on Nov. 2.
The months-long campaigns to get the ballot questions approved allowed proponents also to build political machinery -- to work through churches, and to beef up the ranks of volunteers, donors, and potential Bush voters. Republicans often lament that in 2000 about 4 million evangelical Christians, who would have been Bush voters, stayed away from the polls. This year, gay marriage is the issue that church leaders and political organizers cite most often when urging those Christians to vote.
"No other social issue has been this important in the national campaign," said Tony Perkins, president of the Family Research Council, which has been pushing for a federal constitutional amendment.
The issue has united Protestants and Catholics, and sent some black churches in Bush's direction. In conversations with the most passionate Bush supporters across the country, gay marriage quickly emerges as a top concern.
Bush's use of the issue has angered gay activists, including the Log Cabin Republicans, a gay group that endorsed Bush in 2000 but has refused to do so this year because of his stance on the amendment.
"President Bush is absolutely using gay marriage as a political wedge," said Cheryl Jacques, president of Human Rights Campaign, a gay advocacy group that has endorsed Kerry. "This is the classic politics of division and distraction. He is distracting voters from talking about jobs and the economy and the fact that 45 million Americans don't have health care. It's cold and calculated: 'I only need 51 percent, and I don't care about the rest of America.' "
Bush's stance on the issue brings him more advantages than Kerry's stance does him, said Stephen Hess, a professor of media and public affairs at George Washington University.
"It's part of a trifecta -- gay marriage, abortion, stem cell research -- that tend to fire up the Bush base, without probably having a comparable effect on the Kerry vote," Hess said. Roman Catholic voters, for example, might vote for Kerry, who is also Catholic, and yet they may be ambivalent because "his stands are very much against the doctrines of the church," Hess said.
The issue is tricky for the candidates. Both want to solidify their bases, but neither of them wants to alienate voters at the other end of the spectrum. So Bush is careful to urge tolerance and dignity for gays and lesbians, and Kerry is careful to state his opposition to gay marriage.
Earlier installments of this series can be found at www.boston.com/politics.