Nationwide, most Americans are comfortable with a requirement to have health care insurance. But among Iowa Republicans, a new Bloomberg poll shows health care mandates remain very unpopular.

One area where [Mitt] Romney, 64, is vulnerable is his backing as governor support of a health insurance mandate in Massachusetts that is similar to the one in the federal health-care overhaul passed by Congress last year. More than half — 58 percent — of likely caucus participants said support of such a mandate would “rule out” their backing.

Romney, meanwhile, continues to express his support for a health care mandate — his version of one, anyway — and defend the policy rather well. Alec MacGillis yesterday called Romney, “Obamacare’s Most Effective Spokesman.”

Now, I’ll admit it’s been a while since I worked on a political campaign, but it seems to me that if Republican voters consider health care mandates a deal breaker, and Romney supports health care mandates, then maybe Romney’s Republican rivals might want to focus some attention on this?

I still have no idea why the GOP field is giving Romney a pass on health care. The former governor’s health care included an individual mandate forcing taxpayers to purchase insurance; it provided benefits to immigrants who entered the country illegally; and it covers abortion — and somehow, this hardly ever comes up in the middle of the GOP primary contest. A year ago, the right was saying Romney wouldn’t even be considered unless he renounced and apologized for his health care law, and now, it’s effectively become a non-issue.

Jonathan Bernstein recently argued that Romney’s GOP rivals are “blowing it.” I agree.

How could Romney run for the Republican nomination after providing the blueprint for the health care law that the GOP hates with the heat of a thousand suns? How could Republican voters condemn government health care mandates as the most offensive policy in American history, and then nominate for president the only governor in America to impose a health care mandate on his constituents?

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.