THE venue, gay-friendly San Francisco, is a predictable one for a legal challenge that may lead to the legalisation of gay marriage in America. But that hardly detracts from the drama of the trial of Perry v Schwarzenegger, which began on January 11th. Pitting two gay couples (including Kristin Perry) against the state of California (represented by its governor, Arnold Schwarzenegger), it is a federal review of whether Proposition 8, a Californian voter initiative of 2008 that outlawed gay marriage in the state, is constitutional.

Whatever the outcome, the case is likely to end up in the Supreme Court. But first the trial will do two things: it will establish for the first time a body of expert evidence which appellate courts can use to evaluate claims by supporters and opponents of gay marriage. And it will blur the existing partisan divide on the issue between conservatives and liberals.

This is thanks to the unusual pairing of lawyers who are arguing in favour of gay marriage for the plaintiffs. David Boies, a well-known liberal who acted for Al Gore in his 2000 court fight against George Bush over the presidency, is an unsurprising choice. But his partner, Theodore Olson, is a prominent conservative and was Mr Boies's courtroom adversary in 2000. Now he has teamed up with Mr Boies to argue for legalising gay marriage based on what he considers an arch-conservative interpretation of the constitution.

The defence is hoping to establish the right of voters to amend the constitution of their state. California's Supreme Court, six of whose seven justices were appointed by Republican governors, ruled in May 2008 that a ban on same-sex marriage violated California's constitution. This ruling galvanised opponents of gay marriage to put Proposition 8 on the ballot. It passed with 52% of the vote, and the state Supreme Court upheld it.

America's constitution, however, does not allow rights to be stripped from its citizens by majority vote. The federal Supreme Court ruled in 1996 that any laws motivated by homophobia (or other bigotry) are unconstitutional. The motivation behind the Proposition 8 campaign has thus become an issue. Scholars will testify about homophobic discrimination.

They will also be cross-examined about other claims against gay marriage. One concerns the sanctity of tradition. But, as Mr Olson has already argued, the fact that something has been custom in the past does not require it to remain that way—otherwise, America would still ban interracial marriage.

Ultimately, Mr Olson is trying to establish that conservatives should welcome, not fear, gay marriage. They revere marriage as a social institution, so the respect of gays for it should be a cause for celebration, not disgust. And there is also the 14th amendment of the constitution, which guarantees Americans equal protection under the law.