It’s all about conservative voter turnout, says Nate Silver:
Imagine that [Michele] Bachmann has won the Iowa caucuses while [Mitt] Romney has taken the New Hampshire primary, and the nomination is essentially up for grabs between them. As the contest shifts to a key state like Ohio or Pennsylvania, suppose that conservative Republicans split 60-30 in Ms. Bachmann’s favor (with a few voters opting for a hanger-on like Ron Paul), while moderate Republicans go 80-15 for Mr. Romney. Who is going to win?