She's changing her position because the vote was going to fail anyway. Political posturing at its worst. Much like the introduction of this bill right on the heels of the Senate voting for Amnesty.
It's frustrating to watch us being played for fools... the Senate voted 49-48: way below the needed 67.. Is the WH going back and trying to rework a stategy to build up support? NO. The bill is going to a vote in the House, where everyone will get up and bluster for the home town audience, take a vote which will be far short of the needed 2/3 super majority, and the "Conservatives" will be able to go back to their districts, claim they voted for the sanctity of marriage (while a good many of them visit their mistresses), and continue to have no serious longterm strategy to succeed on this issue.
Sorry, I'm feeling very cynical today..