During the 2000s U.S. mortgage borrowing experienced its most volatile cycle in the postwar record, with mortgage debt more than doubling between 2000 and 2008 before declining by more than 10% over the next five years. The consequences of the boom and bust for both borrowers and the wider macroeconomy were significant, with millions losing their homes to foreclosure or their jobs to the ensuing deleveraging-driven recession. Recent research has focused on variations in credit supply as a primary determinant of both the boom in mortgage borrowing and subsequent collapse, as well as the concurrent rise and fall of residential real estate prices and employment. In the wake of the Great Recession many have called for countercyclical policy intervention in the mortgage market, both to restrain over-leveraging during booms and to provide additional access to refinancing credit during busts. Moreover some analysis has placed the blame for the volatile U.S. credit cycle on the policies of Fannie Mae and Freddie Mac, the two largest government-sponsored enterprises, which have been labeled as excessively risky, actively destabilizing, and regressive. Nevertheless, though many have called for their reform these two agencies appear to be a continuing feature of the U.S. housing finance system and are currently well-positioned to implement countercyclical credit supply policies. In my dissertation I propose a novel countercyclical policy intervention by the government-sponsored enterprises and analyze its impact on mortgage borrowers.