Not running: Mike Huckabee, the 2008 runner-up; John Thune, the likeliest candidate from the Senate, the body that has produced the out-party candidate in 2008, 2004, and 1996; Mike Pence, who could lay as much claim as anyone to represent the conservative movement; and Haley Barbour and Mitch Daniels, effective two-term governors with impressive D.C. experience as well.
It would be unfair to call the current field a vacuum. But it doesn't exactly represent an overflowing of political talent. And insofar as politics abhors even a near-vacuum, others are bound to get in. I now think the odds are better than 50-50 that both Rick Perry and Paul Ryan run. I also now think they (and others—Sarah Palin, Chris Christie, John Bolton) may not feel they have to decide until after Labor Day—or maybe even until October or even November. The field could well remain open and fluid until Thanksgiving.