Friedersdorf takes a close look at those who have shied from the commitment: 

[I]t's a sign that there is little enthusiasm for any candidate among a diverse array of influential movement conservatives, and that doesn't portend well for the eventual Republican nominee. He'll have a difficult time generating enthusiasm, and may be tempted to try compensating as John McCain did: by making a risky VP pick to drum up support (or give ideologues cover for offering it) — though hopefully the right has at least learned that lesson in the last four years.