There is a common assumption that the Republicans nominate the candidate who is "next in line." By any important metric, that candidate is Rick Santorum. In the 2012 GOP nomination contest, he got the second-highest number of delegates (267). He got the second-highest number of votes in primaries (3.9 million). He also led Romney in national polls for a time in February 2012.
And yet he has not scored well enough in the polls to make tonight's prime-time debate. He is laying off staff, and shows no signs of resurgence.
Let us bury the "next in line" theory. It rests on half a dozen examples, which hardly make for an iron law. As we point out in After Hope and Change, the "next in line" candidate for the 2000 nomination was Steve Forbes, who had scored surprisingly well four years earlier but who could not even come close to George W. Bush. In 2012, the Republican who was next in line was arguably Mike Huckabee, who had won more delegates than Romney in 2008. He did not even bother to run.