The graph below shows the performance of the S&P 500 during election years. We can see that there is an average of an 11.28% overall.

It is in the benefit of the ruling government to have a successful stock market performance to incline the public opinion so they get re-elected. Hence, they pressure the FED for favorable measure like QE and lowering rates to achieve this outcome.

This data has been extracted from the following page: CLICK