WASHINGTON -- Negotiations over a must-pass bill to avert a government shutdown have stumbled on an unlikely roadblock that has nothing to do with government spending or federal debt. Rather, it would hand heftier profits to a handful of private equity firms by amending a Depression-era bankruptcy law.

Listen to HuffPost discuss the controversial rider in the latest episode of the politics podcast 'So That Happened,' embedded above. The fun begins at 22:30.

Senate Banking Committee Chairman Richard Shelby (R-Ala.) nearly succeeded in attaching the private equity aid to a highway funding bill earlier this month, but the measure was stripped at the last minute amid opposition from liberal Democrats. Senate Minority Leader Harry Reid (D-Nev.) is now the lead champion of Shelby's provision, and is seeking to include some version of it in a bill to fund the federal government.

The policy rider would help two powerful financial firms -- Apollo Capital Management and TPG Capital -- exact financial concessions from other investors involved in a bitter battle over the fate of the Caesar's Palace casino empire. The New York Times' Gretchen Morgenson reported on the dispute in the fall of 2014, noting that Apollo and TPG were facing off against creditors that included a pension fund for California teachers.

Apollo and TPG own Caesar's Entertainment Corp., a casino empire that operates, among many properties, the famed Las Vegas gambling house. Caesar's has faltered under a heavy debt burden incurred by the complex deal that allowed Apollo and TPG to take over the company. Those two owners now want to shed that debt without formally declaring bankruptcy, since a bankruptcy court would require them to take heavy losses.

Their efforts run afoul of the 1939 Trust Indenture Act, which bars companies from rewriting the terms of bond deals without the consent of the bondholders. The rider being pushed by Reid and Shelby would allow Apollo and TPG to proceed with plans to force financial concessions from other investors.

The effort has created consternation among liberal Democrats in Congress and many academic securities law experts . On Thursday, 20 House Republicans joined the dissent, writing a letter to Speaker Paul Ryan (R-Wis.) and Majority Leader Kevin McCarthy (R-Calif.) urging them to reject the maneuver being backed by Reid and Shelby.

"[The] amendment would suddenly disrupt the settled expectations of creditors who in good faith relied upon long-established features of federal securities law," the letter reads. "This retroactive proposal has not been the subject of public hearings."