When the President and Senate are Republican, and the House Democratic, the S&P 500 averages a 13.7% return. When everything is reversed, which may happen at this midterm, returns average 13.6%. When the President is a Democrat and the Capitol Republican, the S&P 500 averages 13%. When D.C. is all Republican, equities average 12.9%, and when it’s all Democrats, 9.8%. With a Republican president and Democratic Capitol returns average 4.9%.