Conservatives and business organisations also join forces in urging court to rule against law that bans same-sex marriageThe Obama administration is to make its strongest declaration yet in support of same-sex marriage, filing an eleventh-hour brief in support of a supreme court challenge to California's law banning such unions.Advocates for marriage equality have been urging the president to support the two gay couples who are bringing the case, and an administration confirmed on Thursday that it would file a "friend of the court" brief before the deadline at the end of the day.The case, due to be argued on 26 March, will hear whether California's 2008 law, known as Proposition 8, is constitutional.Briefs in support of the case have already been filed by a diverse group of Republican and conservative leaders, some of America's leading businesses, 15 states and equality groups.The Obama administration is not required to file a brief, and earlier indications suggested that he would not, despite speaking in his inaugural last month of gay men and lesbians being treated equally "under the law".