It requires 2/3 vote of both Houses of Congress. There is not enough support in the US Senate to even bring this to a vote currently.
Also, libertarian leaning Republicans will not support it either. It's not going to even come up for a vote for a long, long time. This was, IMO, a calculated risk on President Bush and Rove's part, they figured he could bring up the call for an ammendment, at the same time knowing he would probably never have to act on it, and risk alienating the minority that support gay unions. Rove is good at watching all of the angles before having the President announce a position or propose legilation.
but there is enough support in America to use this amendment to un-elect libs like Dashcle and others which will get us the judges we want, thereby protecting marriage