Whatever happens in Thursday’s debate and Saturday’s straw poll in Ames, the Republican field is likely to be narrowed. No candidate will come out of Ames the same as he or she went in. Some may not come out at all. And one will emerge as the top challenger to Mitt Romney, whose decision not to compete in the straw poll turned the voting largely into a contest between Tim Pawlenty and Michele Bachmann.

The week will also be key for Rick Perry, the Texas governor who’s been weighing a run, but won’t be on the Ames ballot. The less strength everyone else in the field shows, the wider the opening for him to get in.