President Obama keeps hinting that he might become the first sitting U.S. president to support same-sex couples and their effort to get married. The president said his views on gay marriage are "evolving" and noted that he knows that, for supporters, civil unions aren't "enough." Still the president's change of heart doesn't mean the White House is likely to lead a national campaign to legalize gay marriage. There are two stumbling blocks: Most marriage laws are written by the states. Any attempt to repeal the Defense of Marriage Act by Obama would face stiff resistance from a Republican-led House of Representatives.