UPDATE: The text of Romney's speech withdrawing him from the race. He says he's withdrawing for the sake of party unity and to ward off a potential Obama/Clinton presidency:
I disagree with Senator McCain on a number of issues, as you know. But I agree with him on doing whatever it takes to be successful in Iraq, on finding and executing Osama bin Laden, and on eliminating Al Qaeda and terror. If I fight on in my campaign, all the way to the convention, I would forestall the launch of a national campaign and make it more likely that Senator Clinton or Obama would win. And in this time of war, I simply cannot let my campaign, be a part of aiding a surrender to terror.
This is not an easy decision for me. I hate to lose. My family, my friends and our supporters… many of you right here in this room… have given a great deal to get me where I have a shot at becoming President. If this were only about me, I would go on. But I entered this race because I love America, and because I love America, I feel I must now stand aside, for our party and for our country.