WASHINGTON – Two rivals for the 2012 Republican presidential nomination are attacking former Massachusetts governor Mitt Romney where it hurts the most: health care. The moves mark the start of the race to challenge Romney, the clear frontrunner, by highlighting his key vulnerability with the conservative base.

Former Minnesota governor Tim Pawlenty, who is seen as perhaps the most electable alternative to Romney, is poised to air a new ad in Iowa boasting that he "passed health care reform the right way: no mandates, no takeovers." It's an obvious jab at Romney's universal health care law in Massachusetts, essentially the same thing as the Affordable Care Act but on a state level.

Discussing the ad, Pawlenty told Politico he intends to go after Romney on health care, and stress the similarities to President Obama's health law by using words like "Obamneycare" and "Robamacare."

Also attacking Romney on the issue is Jon Huntsman, the former Utah governor who announced his 2012 candidacy Tuesday. Huntsman told ABC News that Romney "has little credibility" on health care "based on the model that was created in Massachusetts."

"It's creating, I think, a lot of uncertainty in the business world and causing a lot of people not to get active and involved and invest at a time when we need investment desperately,” Huntsman said.

That even Pawlenty and Huntsman, two relative moderates in a very conservative GOP field, are criticizing Romney's health care law reflects just how unacceptable the law has become to Republican electorate. Conservative activist groups are even invoking it to wage campaigns against the ex-governor.

For all the conservative gripes, Romney's Commonwealth Care law has been a clear success in terms of insuring its residents. Massachusetts has an uninsured rate of just 5 percent, notably lower than the rates in Pawlenty and Huntsman's home states.

The attacks nevertheless underscore what will be a key dynamic to watch in the 2012 race: whether one of Romney's rivals can chip away at his vulnerabilities and mount a threatening challenge to the man leading in name recognition, fundraising and organization.

Romney has stood by his Massachusetts plan but has pledged to repeal the Affordable Care Act, arguing that the idea is acceptable on a state level but unconstitutional on a federal level.

Watch Pawlenty's ad below, via YouTube.

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