Are you ready for a replay?

Mortgage-backed securities—and the bankers who loved them—wreaked havoc last year, helping to pitch us into the deepest downturn since the Great Depression. Are you ready for a replay?

Gird your loins. The signs are growing that there's a new Wall Street gold rush under way—for those complex bundles of mortgage loans that fueled banks' profits between 2005 and 2007. This year, prices for mortgage-backed securities are rocketing as federal stimulus dollars flood the market. But the difference with this "boom" is the center of gravity has shifted: from giddy, cowboy bankers to the Federal Reserve. The Fed is so eager to save banks, create a demand for these securities, and stabilize the housing market that it's taking troubled loans and mortgages onto its own books. The problem is the Fed may be in well over its head.

The Fed is cleaning up the old mortgage securities in the market—mostly old residential mortgage loans backed by Fannie Mae and Freddie Mac. But it will soon be on the hook for new ones, too, as troubled commercial mortgages are expected to fail en masse in a crash. The institution has already received $2.3 billion in requests to buy commercial mortgages. Indeed, investors are so eager to dump their commercial mortgage-backed securities on the Fed that they have spurred an outcry against Standard & Poor's, which has said it may tighten its ratings requirements to keep the more problematic loans out of government hands. Put simply, the holders of such securities don't want anything to stand in the way of getting on the federal gravy train.

Both types of assets are creating a shadow boom in unworthy debt, based on the same excessive leverage and questionable financial judgment of the last credit bubble. Plus the Fed is making some of the same mistakes as banks did in 2005-07. The banks forgot they were in the "moving" business-of underwriting mortgage-backed securities—and got into the "storage" business of keeping those securities on their books. That's where the Fed is now. It has not yet articulated an exit strategy to dump up to $800 billion of mortgages from its balance sheet.

And if you thought U.S. banks holding all those sketchy mortgages was a bad idea, wait until you see what happens when the center of our country's money supply is saddled with bad debt. The Fed could bail out the banks; no one can bail the Fed out.

It's partly a matter of sheer dollars. The Fed has dug itself in deep, spending $64 billion over the last four weeks and $741 billion this year as it plans to purchase $1.25 trillion of securities backed by agencies like Fannie Mae and Freddie Mac.