Here are my “no inside sources” speculations on what drove Bush to pick Alito:
a) Bush obviously had to get his base, especially those in the organizational wing of the Republican party back on side. Based on all of the initial blogging today, he succeeded completely–conservatives seem to have gone from “time to jump ship” to “all hands on deck” almost immediately.
b) The failed Miers nomination shows the downside of trying to go under-the-radar. While this may have worked in terms of blunting Democratic opposition, the firestorm over Miers shows that the base is no longer willing to go along with it.
c) If we assume that the White House is not a unified actor on nominations (a reasonable assumption), then the failed Miers nomination must have given the “whole hog” contingent in the administration leverage over the “half a loaf” types. The former, who could previously have been stigmatized as “ideologues,” now had a reasonable claim to being more pragmatic than the latter. More to the point, the folks behind the Miers nomination (including Harriet Miers!) would have had a very hard time being taken seriously in their predictions of what would happen with the nomination of their preferred candidate. This would have given the hardliners pretty free room to push whoever they preferred.
d) Connected to (a), Republicans may be worried about turnout in the 2006 Congressional races. Rallying the troops around this nomination gets the base back on side, reminds them why they’re Republicans, and leads them to believe that Bush will put up another Alito-like pick if Bush gets another vacancy in the last half of his term–thus making it imperative to hold the Senate in 2006.