For opponents of same-sex marriage, it has been a potent and constantly repeated talking point: Though the courts or the legislatures of some states have given gay and lesbian couples the right to marry, wherever it has appeared on the ballot, voters have rejected it.
Gay rights advocates hope that after Tuesday's election, no one will be able to say that any more.
With public opinion shifting in their direction, rights groups see a good chance of victory in at least one or two of the four states where same-sex marriage is a ballot item this year.
Polls indicate a solid lead for supporters in Maine and a lesser one in Washington state, while the races in Maryland and Minnesota are about even, with the opponents apparently gaining.
Even a single victory "will be a turning point," said Chad Griffin, president of the Human Rights Campaign, a national group promoting gay rights that has directed $5 million to the four marriage battles.
Six states and the nation's capital have legalized marriage for gay and lesbian couples. But in 30 states, in constitutional amendments, voters have limited marriage to a man and a woman; in addition, same-sex marriage has been blocked in referendums like those in California in 2008 and Maine in 2009.
Opponents of broader rights for gay and lesbian couples are mounting a final-week barrage of advertising and telephone appeals, warning undecided voters that "redefining marriage" would force onerous changes in schools, businesses and churches. Rights groups denounce those messages as misleading scare tactics.
"Our ads have started in earnest, and they will be airing more heavily in the final week," said Frank Schubert, a California-based consultant who is managing all four state campaigns against same-sex marriage, mainly with financial aid from the National Organization for Marriage and affiliates of the Roman Catholic Church.
Perhaps the most expensive campaign is in Washington state, where supporters of Referendum 74, to endorse same-sex marriage, have raised nearly $11 million, including $2.5 million from Jeff Bezos, the founder of Amazon.com, and his wife, Mackenzie, and $600,000 from Bill and Melinda Gates.
The opponents have raised $2.4 million, including $1.1 million provided by the National Organization for Marriage.