As he wrote in 2004, “Study after study have shown that Fannie Mae and Freddie Mac, despite full-throated claims about trillion-dollar commitments and the like, have failed to lead the private market in assisting the development and financing of affordable housing.”

After the crisis, his tune changed considerably  as did that of many other Republicans, who tended to follow his intellectual lead on this issue. Now, he said, it was government policy aimed at increasing homeownership that essentially forced the private sector to make bad subprime loans.

And Fannie and Freddie, with their enormous power in the securitization market, were the government’s vehicles in leading Wall Street and the other market participants down this garden path. “Fannie and Freddie were in competition to reduce underwriting standards,” Mr. Wallison told me when I spoke to him this week. This of course directly contradicts his criticism of six years ago, but never mind.

It is easy enough to understand why Mr. Wallison and his fellow Republicans would find this a comforting prism through which to view the financial crisis. If government housing policy is the chief culprit, it means that the market itself is off the hook. See, it was all the fault of wrong-headed government policy! Blaming it all on Fannie and Freddie also gives the Republicans a nice cudgel with which to beat Democrats, who for years defended the G.S.E.’s from even the most muted criticism.

The Republican document issued earlier this week did little more than regurgitate this theory of the case. “Subsidizing mortgages through the G.S.E.’s was a particularly expedient way to increase the homeownership rate,” they write at one point. At the same time, they tread lightly over the culpability of other nongovernmental culprits like the credit ratings agencies and Wall Street itself.

The only problem with Mr. Wallison’s theory is that it’s not, as they say, reality-based. Anyone who has looked at the role of Fannie and Freddie will discover they spent most of the housing bubble avoiding subprime loans, because those loans didn’t meet their underwriting standards. (Indeed, for most of their existence, Fannie and Freddie didn’t so much meet their affordable housing goals as gamed them.)

When Fannie and Freddie finally did get into the business, it was very late in the game. But the motivation wasn’t pressure from the government; it was pressure from the marketplace. You see, the subprime companies and Wall Street had long used subprime loans as a way to do an end-run around Fannie and Freddie. By the mid-2000s, subprime underwriting and securitization had become so profitable  and such a large part of the overall mortgage business  that Fannie and Freddie felt they had no choice but to dive in. In other words, the G.S.E.’s were reacting to the realities of the market, not to the government. They were worried about losing market share.