Here's a rundown of Romney's past strategic ambiguity on whether he ever thought a national health insurance mandate was a good idea:
http://nymag.com/daily/intel/2012/03/romneys-health-care-evasions-a-history.htmlApparently, according to Noam Scheiber's reporting here:
http://www.tnr.com/article/politics/magazine/101650/romney-opponents-health-care?passthru=NzUxYzFjMTk4YzY2MGY2NjY1MTRiNWIxYzViMjQ4YzEHuntsman, Pawlenty, and Perry all had oppo for Romney on health care (e.g., "video footage of Romney touting the health care plan he enacted in the state as a “model” for the country, as well as a USA Today op-ed Romney authored making the same point"), but they never used it.
They didn't want to introduce themselves to voters by going negative, so they held this stuff in reserve, for when they'd be in one-on-one contests with Mitt....a day which never came. They all dropped out, and the ABR mantle fell to Gingrich and Santorum, two candidates running shoestring campaigns, who never had dedicated oppo researchers like this.