The potential liability facing bankers arises from the $2 trillion in subprime, alt-A and option-adjustable rate mortgages that they underwrote and sold to investors, mostly as mortgage-backed securities during the home-lending boom of 2005 to 2007. The losses on the mortgages will be horrendous before the dust settles—over $700 billion on these and other so-called nonagency mortgage securities, according to New York mortgage-research specialist and broker Amherst Securities Group.

And now investors—from the federal housing giants Fannie Mae (NYSE: FNMA.OB - News) and Freddie Mac (NYSE: FMCC.OB - News) to major bond managers like closely held Pacific Investment Management and BlackRock (NYSE: BLK - News) —are fighting back. They are seeking to put back the mortgages to the banks from whence the investment flotsam came and force the banks to eat much of the mortgage losses.

The argument hinges on the arcane contract principle of representations and warranties, known as reps and warranties in legal jargon. Namely, did the mortgages go bad because of the unanticipated nationwide collapse in home prices (a so-called exogenous factor) or are the banks responsible for the mess because they "misrepresented" to the mortgage purchasers the shoddy quality of the mortgages they put in securities and pools?

lready some of the buyers have enjoyed a modicum of success in their putback efforts. Fannie Mae and Freddie Mac have managed to return over $13 billion in defective mortgages and are gearing up to do even more. Before it's all over, the banks may have to swallow more than $30 billion in losses from Freddie and Fannie putbacks alone, according to an estimate by the Washington, D.C., mortgage-research boutique Compass Point Research & Trading. That's because no bank in the mortgage business can afford to play hardball with secondary market behemoths like Fannie and Freddie, the government-sponsored enterprises (GSEs) that own or guarantee about half of all home mortgages in the U.S.