"President Obama is moving us forward," the ad says. "Mitt Romney would take us back."
In a sense, Obama is the one going back, returning to a position he took as a political novice in 1996. But the same changes in public opinion that made it thinkable for him to stop equivocating on gay marriage present a challenge to Romney as he repositions himself for the general election.
Sixteen years ago, when Obama supported "legalizing same-sex marriages" as a candidate for the Illinois Senate, a Gallup poll found that only 27 percent of Americans agreed with him. According to a Gallup poll conducted this month, that number has risen to 50 percent.
A new CBS News poll indicates that support for legal recognition of gay couples (not necessarily "marriage") is even higher. Thirty-eight percent of respondents said that "gay couples should be allowed to marry," while another 24 percent said they should be "allowed to form civil unions." Only 33 percent favored "no legal recognition." Surveys during the last few years have yielded similar results.
In short, "no legal recognition" for gay couples clearly has become a minority position, which poses a problem for Romney. The former Massachusetts governor has opposed same-sex marriage since the beginning of his political career, and he favors a constitutional amendment that "defines marriage as a relationship between a man and woman."
The Obama campaign's video implies that Romney -- unlike Obama's Republican predecessor, George W. Bush -- also opposes civil unions, but that is not true. Since running for governor in 2002, Romney has said he supports "domestic partnerships" for same-sex couples that include "the potential for health benefits and rights of survivorship."
What else they might include is not entirely clear, and this is the tricky part for a candidate trying to keep social conservatives happy without alienating swing voters by seeming intolerant or insensitive to the problems gay couples face because of their unequal legal treatment.
Romney's idea of domestic partnerships clearly does not go as far as the civil unions that Obama favored until last week (which he said would provide "all the rights" of marriage). "I do not favor marriage between people of the same gender," Romney said after Obama's announcement, "and I don't favor civil unions if they're identical to marriage other than by name."
But contrary to the Obama campaign's video, Romney does support shared health plans as well as joint adoption. "If two people of the same gender want to live together, want to have a loving relationship and even want to adopt a child," he said on Fox News last week, "in my view that's something that people have the right to do."
Already this is dangerous territory as far as social conservatives are concerned, which explains why Romney's campaign later insisted he was only explaining what Massachusetts and many other states allow. "He thinks a traditional family is far better for children," a spokeswoman told CNN, but "he acknowledges it's a state issue" and "did nothing to change it" as governor of Massachusetts.
As that whipsawing statement suggests, federalism will get Romney only so far, especially since he has chosen to nationalize the issue by calling for a constitutional ban on gay marriage. If the ban does not apply to civil unions, it will not stop states from allowing legal arrangements "identical to marriage" but for the name, which Romney says he opposes. But if the federal government tries to prevent those, states won't really be free to "make decisions with regard to domestic partnership benefits," the approach he says he favors.
Romney is not the only Republican with conflicting impulses on gay marriage. In the CBS News survey, 70 percent of Republicans supported a constitutional ban, while 63 percent said the issue should be left to the states.