The econ team at Goldman Sachs makes some good points in a research note about the wisdom of bailing out Fannie Mae and Freddie Mac, what it might cost taxpayers, and why we should not be weirded out by the idea. Let me paraphrase:
1) Fannie and Freddie have $5.3 trillion in liabilities.
2) Unlike most federal liabilities, agency debt and mortgage-backed securities are backed by financial assets—ultimately, mortgages. Indeed, this does pose credit risk to the government.
3) But consider that the federal government already explicitly guarantees credit in several sectors of the economy, including education, housing, agriculture, transportation, and certain businesses. Much of this debt is secured—for instance, mortgages insured by the Federal Housing Administration. But some is not, and many of these guarantees have default rates significantly higher than the prime mortgages in the government-sponsored enterprises' portfolios.
4) Cumulative losses on the GSEs book of business may total $53 billion. Even if the federal government were to incur these costs, they are one of many factors affecting the fiscal picture, and hardly the dominant one at well under half a percent of GDP.