This Cato Institute article Taking Stock of the Parties analyzes the stock market to determine which political party is better for creating wealth, as reflected in stock prices.

Over the last quarter of a century when the Republicans controlled both houses of Congress, the stock market rose by an average of about 20 percent per year. When the Democrats controlled both houses of Congress, the stock market only rose at an average annual rate of 6.9 percent for the Dow Jones and a tepid 5.1 percent for the Standard and Poor 500.

When one party controlled one house and the other party controlled the other house of Congress, the growth in the stock market was significantly less than when the Republicans controlled both houses (15.6 percent for the Dow Jones and 12.7 percent for the S&P), but significantly more than when the Democrats controlled both houses.

There is a natural tendency for people to focus on the party of the president who is in power; but, in fact, the Congress is far more important to markets because it decides how to, and how much to, tax and spend.

In conclusion:

History shows if you want your stock portfolio to increase at a rate of 20 percent per year, make sure Republicans control Congress; but if you want your portfolio only to increase 6 percent per year, go with the Democrats.