The plot has thickened yet again in what Paul Krugman has called The Case of the Mystery Study.

Congressional Democrats have intensified the behind the scenes pressure on McKinsey and Company to release the methodology of their study finding that large numbers of businesses plan to drop insurance for their employees because of the Affordable Care Act, I’m told. But the company is still refusing to reveal key details about the study’s methodology, which has made it impossible to verify the study’s accuracy or integrity.

House Dems have privately pushed McKinsey to explain why McKinsey can’t release the information, given that McKinsey has confirmed to them that it did not do the study for any outside client. McKinsey did the study for its own purposes, the company informed Democrats.

But in a bizarre development, the firm has continued to tell them that the information was proprietary and that it won’t be released, I’m told.

This has Dems asking why the firm won’t release the study’s methodology, given that there’s no outside client who paid for the study and is demanding that it remain confidential. As the White House points out, its findings are at odds with other studies conducted by independent organizations.

“Their findings are so off base that we naturally asked to see who they surveyed and what they asked,” Josh Drobnyk, a spokesman for Democrats on the House Ways and Means Committee, tells me. “The fact that they are not releasing that information obviously calls into question the results.”

House Dems intend to ratchet things up further in the days ahead: They will be sending a formal letter to McKinsey requesting the release of the study’s methodology, sources tell me, which is likely to generate a whole new round of media scrutiny on the company’s refusal to divulge the data.

All this comes as House Republicans are explicitly using McKinsey’s study as a political weapon against the Affordable Care Act. John Boehner issued a statement citing the study as proof that “businesses planning for the onslaught of ObamaCare taxes, mandates, regulations, and penalties have two choices: stop offering health care for their employees, or eliminate full-time jobs and keep wages low.”

As Steve Benen notes today, this study is now “becoming a political weapon,” and the GOP leadership is now “alerting the public to the McKinsey report as if it’s reliable,” even though we have no way of evaluating it.

With Dems set to press the case publicly in the days ahead, this story is only likely to get bigger.