McCain's housing plan

McCain criticized Obama for being associated with Fannie Mae & Freddie Mac, as if the institutions were criminal and Obama was an accomplice. F&F's mission was to create markets to facilitate mortgage selling. Democrats liked them because they helped create "affordable housing" (through sub-prime loans). So there's a valid argument to be made that the whole affordable housing / ownership society ideology was wrong, that F&F contributed to the bubble and the securitization that caused this financial crisis.

Yesterday in the debate, McCain proposed a $300 billion federal program to buy home mortgages and convert them to lower fixed-rate mortgages, so people who took bad loans can stay in their homes. "Will it be expensive?" he asked rhetorically yesterday. "You bet it will."

So I'm confused: how is that different from what Fannie & Freddie did? The federal government would once again be subsidizing or encouraging home ownership through sub-prime mortgages. The government would in effect subsidize the loans at the difference between their current rate and the new, lower rate. Real estate prices would again be artificially propped up by the government. How does that square with McCain's blistering critique of F&F? I don't get it.