"How have the DJIA, NASDAQ and SP 500 historically performed in the second half of a president's term?"
Instead of just responding to the reader, I thought that I would make the answer into its own post.
I performed the calculations for the post-WWII era (1946-2009). Our S&P 500 data goes back to 1950, while our NASDAQ data goes back to 1971.
Here is the raw data:
DJIA
Avg. Annual Performance in First Half of Presidential Term: +4.91%
Total Number of Years: 32
Avg. Annual Performance in Second Half of Presidential Term: +10.46%
Total Number of Years: 32
NASDAQ
Avg. Annual Performance in First Half of Presidential Term: +4.43%
Total Number of Years: 19
Avg. Annual Performance in Second Half of Presidential Term: +18.14%
Total Number of Years: 20
S&P 500
Avg. Annual Performance in First Half of Presidential Term: +4.47%
Total Number of Years: 30
Avg. Annual Performance in Second Half of Presidential Term: +12.18%
Total Number of Years: 30
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You can speculate as to the reasons behind this phenomenon, but the data is pretty clear - the markets perform markedly better in the last two years of a presidential term compared to the first two years.
Sources: Historical Dow Data, Historical Nasdaq Data, Historical S&P 500 Data
For how stocks have done after every midterm election since 1946, see Stocks After Midterm Elections: 80 Years of Data.