The S&P500 performs best on average under a Democrat president, data from Kleinwort Benson has shown. Since 1945 the index under Republican presidents has returned 6.0%  per year on average, compared with a return under Democrat presidents of 12.2%. However, it performed best under Gerald Ford (Republican) with a 25.1% average annual return but worst under Richard Nixon with a 5.9% decline.

Average annual returns:

Democratic

Truman 14.5%
Kennedy 10.7%
Johnson 8.0%
Carter 7.0%
Clinton 15.9%
Obama 13.4%

Republican

Eisenhower 11.8%
Nixon -5.9%
Ford 25.1%
Reagan 10.1%
George Bush 12.7%
George W Bush -2.9%