Senate Homeland Security Committee Chairman Ron Johnson is threatening to subpoena a federal agency for information on an Obama-era policy that allows members of Congress and their staff to get around a key ObamaCare requirement.

The policy lets Capitol Hill officials buy health care with a government subsidy on a small-business exchange. Republicans like Johnson have long described this as an ObamaCare exemption, but have struggled to extract details about how it came to be.

Johnson, R-Wis., escalated his push for answers in a letter Wednesday to Office of Personnel Management Acting Director Kathleen McGettigan.

“The American people have a right to know how and why OPM exempted Members of Congress from the full impact of ObamaCare,” Johnson wrote.

Johnson said he had received an “insufficient” response to an August letter, warning: “If OPM does not produce the entirety of the information and materials requested in my August 16 letter by October 18, 2017, the Committee may be forced to consider the use of compulsory process.”

"Compulsory process" is congressional code for subpoenas. Johnson also is seeking transcribed interviews with five officials.

Johnson specifically wants to know about the process used to avoid a provision calling on congressional lawmakers and their staff to buy ObamaCare plans.

TRUMP TARGETS CONGRESS MEMBERS' OWN HEALTH PLANS AFTER OBAMACARE REPEAL FALLS FLAT

Under the Obama administration, OPM made a special decision to categorize Congress as a small business, allowing lawmakers and their aides to buy on the Small Business Health Options Plan (SHOP) exchange. This way, they could keep getting government payments as an employer contribution.

Absent that, they would have been directed onto the individual exchange, without help from an employer contribution.

This could have created a hardship for many low-paid staffers.

But the rule, which took effect in October 2013, has drawn criticism from Republicans.

“The SHOP exchange is intended for employers with less than 50 employees, yet Congress employs more than 16,000 people,” Johnson wrote in his August letter.

President Trump has even threatened to target the special rule, giving Congress an ultimatum back in July.

“If ObamaCare is hurting people, & it is, why shouldn’t it hurt the insurance companies & why should Congress not be paying what public pays?” Trump tweeted back in July. Days prior, he warned he would “end” the “BAILOUTS” for members of Congress if they did not pass a health bill.

Rep. Ron DeSantis, R-Fla., who personally declines the “subsidy,” backed the president’s threat at the time.

“I think the president would be absolutely within his rights to cancel the Obama rule that conferred this subsidy on Congress,” DeSantis said in July.

But the president did not end the subsidy, and Congress has not passed a health care bill. The House approved a plan in May, but the Senate failed to pass multiple versions, with the most recent being the “Graham-Cassidy-Johnson-Heller” plan.

An OPM spokesman told Fox News that they have received Johnson's letter and are reviewing.