It hasn't been formally designated as such, but so far this is the year of gay rights in the US. The signs are many and various. Sean Penn won this year's best actor Oscar for his brilliant portrayal of Harvey Milk, the first openly gay man elected to political office in the US. Since January, the country has had a president who describes himself as a "fierce advocate of equality for gays and lesbians". And within the last few weeks, three more US states – Maine, Iowa and Vermont – have legalised same-sex marriage.

However, the issue is bitterly divisive: witness the $1.5m 'Gathering Storm' TV ad campaign by a group called National Organisation for Marriage, that portrays same-sex marriage as a mortal threat to religious freedom – or the plight of the current Miss California, Carrie Prejean, who last month may have lost her chance to become Miss USA 2009 by speaking out against gay marriage. Something is changing, and not just attitudes. The controversy over gay marriage is no longer a social or 'values' issue. Increasingly it is seen as a matter of basic civil rights.

No one expressed the changing attitudes better than Maine's Democratic governor, John Balducci, a Catholic who had previously supported civil unions but opposed gay marriage: "It's not the way I was raised, and it's not the way that I am."

But, he noted as he signed the state's bill into law, it did not force any church to recognise a marriage, or perform a ceremony with which it disagreed. Rather, "this is a question of fairness and equal protection under the law, and… a civil union is not equal to a civil marriage." So much for the argument that the dispute is one of semantics, that civil union is marriage by another name, and the insistence by the gay lobby that the word 'marriage' is not worth the fuss.

And soon Obama himself may not be able to ignore the fuss. So far, the White House has managed to avoid being drawn into the controversy, maintaining that gay marriage is a matter for individual states to decide. But, inevitably, the focus is switching to the federal level, as activists demand the repeal of the Defence of Marriage Act passed by Congress in 1996 but opposed by Obama, which barred the federal government from recognising gay marriages, and removed from one state any obligation to recognise a gay marriage conducted in another.

Obama has every reason to tread carefully. In 1993, Bill Clinton got off to a terrible start as president when he chose to wade into the 'gays in the military' controversy. Obama already has enough on his plate. If he espouses gay marriage, he risks losing centrist support and creates exactly the sort of distraction that cost Clinton so dear. The same goes were Obama to bow to the pressure of activists and appoint a first openly gay justice to the Supreme Court.

For the moment though, the latter seem prepared to wait – as well they might. Obama likes to play the long game, and if gay marriage is indeed turning into a civil rights struggle, history's lesson is that such struggles take decades to win.