Twenty-one Senate Republicans signed a letter to the Office of Management and Budget Director Sylvia Burwell demanding that she "not authorize the release of any regulations" that would extend subsidies not currently available under Obamacare to union-managed multi-employer health insurance plans, also known as "Taft-Hartley" plans.

The GOP lawmakers said that, while President Obama has ruled out providing the subsidy through IRS rule-making, he has "left the door open for the Department of Labor to issue a regulation favoring labor unions."

Labor leaders have pushed hard for such a ruling, arguing that without it businesses would either limit coverage or pull out of multi-employer plans. Administration officials have resisted the calls but have held talks with labor union leaders on the matter as recently as Sept. 13.

The Republicans' letter points to the mysterious disappearance from OMB's website of a proposed rule from the Labor Department. The posting was titled "Health Insurance Premium Assistance Trust Supporting the Purchase of Certain Individual Health Insurance Policies Exclusion from Definition of of Employee Welfare Benefit Plan." It disappeared from the OMB site shortly after it was posted Aug. 24.

The disappearance was first reported by the trade publication Inside Health Policy in late August.

Also Wednesday, House Education and the Workforce Committee Chairman John Kline, R-Minn., and Rep. Phil Roe, R-Tenn., requested the Labor Department provide all documents related to the proposed rule, as well as "all legal analyses" it has done on the Obamacare "fix" sought by Big Labor.

A committee source cautioned that the panel has only the name of the proposed rule to go on. No text or summary is available. Still, the source characterized the rule's abrupt disappearance as "suspicious."

The Labor Department and the OMB did not respond to a request for comment from the Washington Examiner.

In the senators' letter, they argue that the Obamacare's regulations clearly state that the law's federal subsidies are meant for low-to-middle income Americans without employer-based insurance.

"The fact is that Taft-Hartley union health plans are not exchange-based plans — rather, they are employer-sponsored health plans. Providing union members with a benefit not afforded to non-union employees is grossly unfair to every non-union worker in America who would receive no such special carve out from the health care law," the senators write.

The letter is signed by Sen. Lamar Alexander, R-Tenn., ranking member of the Health, Education, Labor and Pensions Committee; Senate Republican Leader Mitch McConnell, R-Ky.; and 19 other Republican lawmakers.

Unions fear that without the subsidies businesses will pull out of multi-employer plans, limit coverage or provide workers with fewer hours so as to not fall under the law's 30-hour-a-week threshold for an employee to be considered full-time.

Labor leaders are so worried about the impact of Obamacare on Taft-Hartley plans they passed a resolution at their convention last week demanding changes to the law.

One estimate says the action would balloon Obamacare's costs by $187 billion over a decade.