Final passage of the financial regulatory reform bill was shaping up for later tonight (b/n 730p-8pm), but supporters have hit yet another snag that has thrown passage into limbo (again).

Prepare for some Senate weeds, if you dare...

Dems want an amendment authored by Sens Carl Levin, D-MI, and Jeff Merkley, D-OR, to pass that would make regulators force banks to get out of the business of proprietary trading.

BUT -- because Dems are now past cloture (the procedure used to force an end to debate on a bill), a much tighter set of rules for debate is in force. The Parliamentarian would strike down Levin-Merkley if it was standing on its own as an amendment.

Essentially, Levin-Merkley cannot pass WITHOUT being attached to another amendment --- one by Sen Sam Brownback, R-KS, that would exempt auto dealers from the purview of the new consumer agency.

Brownback, however, feels as if he's been "ambushed" (his words to me just now). He says he doesn't support Levin-Merkley - and he doesn't want it paired with his amendment.

Brownback fears (rightfully - acc to sources) that Dems are just doing this so they can get Levin-Merkley approve -- and that they'll ditch the Brownback language in conference.

Dems are saying Republicans are just trying to avoid final passage of regulatory reform.  Remember, only 3 Republicans supporting cutting off debate on this bill.  Republicans have made it very clear, they don't like this bill --- they want the problems at Fannie Mae & Freddie Mac addressed, stronger underwriting rules, a less-intrusive consumer protection agency, and much more.

A senior floor staffer for Minority Leader Mitch McConnell, R-KY, says it's Brownback attached to Levin-Merkley or nothing. GOPers won't allow Dems to grab a replacement for Brownback, so there's a lot of scrambling going on behind the scenes. 

If Brownback simply withdraws his amendment, the Levin-Merkley amendment dies on the spot.

If Levin-Merkley goes down, "That would be an incredible signal of the overall power of Wall Street," Levin said in a floor speech.

A senior aide to Reid just told me they still hope to finish by about 730p/8p. (Which, of course, means 10PM around here usually).