Bush’s visit to Capitol Hill to push his immigration reform bill has, in effect, transformed the vote on the bill into the American equivalent of a British confidence vote. In a parliamentary country, he would have to resign if he lost the vote. Here, he will stay but be slowly twisting in the wind for the next eighteen months.
In a sense, this vote has assumed an importance somewhat akin to the impeachment vote on Bill Clinton, also eighteen months before his terms ended. If the Democrats had deserted him on that vote, he would have had to leave office. If the Republicans desert Bush on this vote, he might as well do so.