WASHINGTON — Federal Reserve Chairman Ben Bernanke urged Congress today to bolster regulation of mortgage giants Fannie Mae and Freddie Mac, and suggested limiting their massive holdings to guard against any danger their debt poses to the overall economy. Bernanke has previously supported efforts to pare the two mortgage companies' huge portfolios. This time, however, he was a bit more specific and recommended that their holdings might be linked to a "measurable public purpose, such as the promotion of affordable housing." The Fed chief's suggestion was contained in remarks delivered via satellite to a bankers meeting in Hawaii. His remarks...