The Washington Post ran an op-ed this morning from right-wing Wisconsin Sen. Ron Johnson and Republican economist Douglas Holtz-Eakin on the Affordable Care Act. The point, not surprisingly, was to trash the law. But guess what they used as evidence while making their case?

A recent employer survey by McKinsey & Co. found that more than half of all American companies are likely to “dump” their workers into the government-run exchanges. If half of the 180 million workers who enjoy employer-provided care wind up in the exchanges, the annual cost of Obamacare would increase by $400 billion by 2021. If the other half eventually follows suit, and all American employees wind up in the exchanges — which we believe is a goal of Obamacare — then the annual cost of the exchanges would increase by more than $800 billion. […]

This isn’t cynicism; this is realism.

This isn’t policy analysis; this is cheap, partisan hackery.

Johnson and Holtz-Eakin, neither of whom are exactly committed to good-faith arguments, are quoting the McKinsey & Company study accurately, but what they neglected to mention is that the study itself is at the center of a burgeoning controversy. As we’ve been talking about for two weeks, the firm’s highly dubious report continues to generate unanswered questions, and McKinsey refuses to disclose its methodology or subject its findings to any scrutiny.

Indeed, it’s worth remembering that a McKinsey insider conceded this week, “Trust me. The survey is not a good tool for prediction.”

But that’s exactly what Johnson and Holtz-Eakin based their op-ed on — the suspicious study’s predictive value.

Several committees and members of Congress are eager for McKinsey to come clean and answer some basic questions: who funded the study, will McKinsey benefit financially from the results, how the survey was structured, how were participants chosen, how were the interviews conducted, how were respondents “educated,” what was the exact wording of the questions, what was the internal review process like, etc.

And the more the McKinsey report is cited as proof by leading officials, the more important it is to understand how the study came to be, why its authors are terrified of scrutiny, and why it’s at odds with all other available evidence.

Paul Krugman appreciates the significance of the controversy.

It’s hard to escape the conclusion that the study was embarrassingly bad — maybe it was a skewed sample, maybe the questions were leading, maybe there was no real data at all. Whatever.

The important thing is that this must not stand. You can’t enter the political debate with strong claims about what the evidence says, then refuse to produce that evidence.

We are, of course, still waiting for major media outlets to care. Some — Reuters and Wall Street Journal, I’m looking in your direction — were only too pleased to note McKinsely’s findings, but have made no mention of the fact that there’s a major controversy underway about whether those findings are reliable.

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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.