Perhaps it’s time for a revised strategy. If you want to be the leading anti-Romney candidate, go after Romney. If you become the most thorough and effective critic of Romney’s record and stances, you won’t have to worry about all of the other ones.

Another possibility is that some folks in the field find Romney’s odds of becoming the nominee so favorable that they don’t want to alienate him, and want to preserve their viability as a running mate or future cabinet secretaries, etc.