The strong decline in 2017 reverses the large spike around the last election. Current values are still elevated relative to the post Financial Crisis average which has been significantly higher than the longer period from 1980-2009.
The caution that many may have had for investing in the stock market during the post Financial Crisis may be the result of this heightened partisan uncertainty. If equity investing is driven by the "animal spirits" of optimism, conflict will dampen risk-taking behavior. This conflict may not be ending, but it may be in the process of normalizing.