Twenty Republican senators are urging the Obama administration not to give unions special relief from an Obamacare tax that other employers, charities and organizations will be forced to pay under the healthcare law.

Most Republicans spent Thursday training their fire on President Obama's decision to offer a "fix" to allow individuals to keep their health plans even if they receive cancellation notices. But a group of GOP senators chose the same day to speak out against administration plans to exempt unions from a new Obamacare tax.

Nearly two weeks ago, the Department of Health and Human Services quietly released a final rule that includes an intention to exempt some union insurance plans from a substantial new tax known as the reinsurance fee.

As part of Obamacare, the tax was supposed to be levied against all insurance plans to share the risk for insurers taking on the sickest patients next year. But unions, which were among the strongest supporters of the Affordable Care Act when it passed in 2010, said the tax would harm insurance plans accessed by more than 15 million union members and would raise costs.

Georgia’s GOP senators, Johnny Isakson and Saxby Chambliss, joined 18 other colleagues Thursday in speaking out against the proposed union carve-out, calling the special treatment “cronyism at its worst.”

In a letter to Sylvia Burwell, director of the Office of Management and Budget, the senators said the fee creates an expense for unions, as well as employers, charities and faith-based organizations who offer health plans outside of the Obamacare insurance exchanges and would not benefit from the tax.

For the year 2014, the fee — $63 per person covered — would mean a multi-million dollar expense for some larger organizations, they wrote, and offering a special carve-out for unions would be unfair to the other companies and organizations that will still be required to pay it.

“The regulation makes no justification as to why union members should be exempted from this fee while other similarly situated organizations (and ultimately, their beneficiaries) must pay for it,” the senators wrote. “To think that the Obama administration would consider such an action that benefits one group over another can only be characterized as cronyism at its worst.”

Along with Isakson and Chambliss, the letter was signed by Sens. Mitch McConnell, R-Ky.; Orrin Hatch, R-Utah,; John Thune, R-S.D.; John Barrasso, R-Wyo.; Richard Burr, R-N.C.; Tom Coburn, R-Okla.; Thad Cochran, R-Miss.; Mike Enzi, R-Wyo.; Jeff Flake, R-Ariz.; Lindsey Graham, R-S.C.; Chuck Grassley, R-Iowa; James Inhofe, R-Okla.; Mike Johanns, R-Neb.; Ron Johnson, R-Wis.; Mike Lee, R-Utah; John McCain, R-Ariz.; Pat Roberts, R-Kan.; and Tim Scott, R-S.C.

Sen. Ted Cruz, R-Texas, also has blasted the administration for the proposed union fix, although he was not one of those who signed on to the letter.