Going back to 1945 with data from about.com, combined with historical stock market data from yahoo finance, I’ve constructed a table showing the relationship between who controls congress (and the white house) versus the growth or decline in the stock market. An image of the table is below. Click table the excel sheet.

Below the table, I point out my findings…but feel free to draw your own conclusions.

Average Stock Market gain during Democratic Congress: +9.8%
Average Stock Market gain during Republican Congress: +30.1%

Average Stock Market gain during Democratic President:
with Democratic Congress support: +13.2%
with Republican Congress opposition: +32.6%
(+18.8% regardless of Congress)

Average Stock Market gain during Republican President:
With Democratic Congress opposition: +6.6%
With Republican Congress support: +25.3%
(+13.3% regardless of Congress)

This is not my opinion. These are facts.

(data for DJIA for each year taken from 1st trading day in that year. Gains are stated for 2 year periods. Half them for annual gains.)