Senate Homeland Security and Governmental Affairs Chairman Ron Johnson has now issued a subpoena for details of the development of the program through which members of Congress and many staffers get health insurance.

The Republican from Wisconsin is following through on pressing the Office of Personnel Management for details on how the Obama administration crafted the policy allowing members and covered staff to get health insurance, with assistance of federal employee contributions, through the small business exchange the District of Columbia set up under the 2010 health care law.

“To date, OPM’s compliance with the Committee’s inquiry has been deficient. In the four months since my initial request, OPM has produced a limited number of responsive documents, made available for in camera review only a small subset of responsive material, and refused to produce some documents specifically identified by the Committee during the in camera review process,” Johnson wrote in a letter dated Friday accompanying the subpoena. “OPM’s refusal to produce these documents, after months of delay, has frustrated the Committee’s oversight efforts.”

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Johnson’s Homeland Security panel has government operations jurisdiction, and his panel has not advanced President Donald Trump’s nominees to be the director or deputy director of OPM.

It has been clear for months that Johnson has been using the OPM nominations as leverage to get the information requested, to no avail. That’s despite the fact a Republican now occupies the Oval Office and Trump has commented from time to time about upending the current arrangement for lawmaker health benefits.

“The American people have a right to know how and why OPM drafted a final regulation that allows Members of Congress and staff to continue to receive an employer contribution, paid by the taxpayer, without authorization in law. The limited information available to the Committee shows that OPM initially believed it could not provide an employer contribution to Members of Congress and staff,” Johnson wrote Friday. “The Committee requires the full universe of material to understand fully OPM’s development of the regulation.”