SAN FRANCISCO (MarketWatch) -- Christopher Cox, chairman of the Securities and Exchange Commission, proposed limits on short selling of shares in Fannie Mae FNM, +0.85% and Freddie Mac FRE, -0.64% on Tuesday, according to CNBC. A spokesman for the SEC didn't immediately return a phone call seeking comment on Tuesday. Short selling is a way to bet against securities. Fannie and Freddie shares have slumped roughly 70% in the past month.