There are two of them. Justin Fox has a good analysis here.
UPDATE: Politically at least, the Cantor plan has a lot of appeal. By insuring these junky mortgage-backed securities, rather than buying them, the government presumably wouldn’t be spending nearly as much money. In fact, it would be getting money from Wall Street, in the form of premiums for this insurance. This scheme would function sort of like GNMA. The very process of insuring these assets would help solve one of the biggest problems: Nobody knows what they are worth.
The problem, at least in the eyes of Treasury Secretary Henry Paulson and Fed Chairman Ben Bernanke, is that, while it would help the situation, it wouldn’t work to stabilize the markets as well as their plan would.
Here’s how it has been explained to me: Last winter and spring, when Treasury and the Fed analyzed a lot of options out there for what to do, they considered this insurance option. They decided that because the insurance option would leave the bad stuff on the bank balance sheets, it wouldn’t give the banks the additional liquidity they need. They also believe it wouldn’t create a market price that can stimulate trading, the way a purchase program would.
I’m not any kind of an expert on this stuff, so I don’t know who is right here. But that, at least, is the rationale on both sides.