news of an unlikely alliance on a controversial topic. Clint eastwood has announced he is joining the side of president oba obama, in one of the most polarizing debates in the nation, gay marriage. Abc's chief white house correspondent jonathan karl explains. Reporter: Less than a year ago, the president still personal little opposed gay marriage. Now, tonight, he is urging the supreme court to strike down california's ban. The move puts president obama on the same page as clint eastwood. So, mr. President -- Reporter: The man with the empty chair who ridiculed him at the republican convention. Both now are part of a broad coalition urging the supreme court to strike down california's proposition 8, which defines marriage as only between a man and a woman. The case against prop 8 is supported by a long list of companies from apple to xerox and a long list of influshl republicans, including four former governors and top edadvis advisers to president george w. Bush. Obama's views on gay marriage have changed dramatically since he was first elected president. Marriage is between a man and a woman. My feelings about this are constantly evolving. I struggle with this. I think same-sex couples should be able to get married. Reporter: At the time of that interview, the president still said the issue should be left to the states. But his evolution continued, and in january, a hint of what was to come. Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law. Reporter: The supreme court will hear the case in less than a month. Possibly deciding for the first time whether there is a constitutional right to marriage. Jonathan karl, abc news, the white house.