1. Market price changes are changes from what was expected before compared to what is expected now. And not even only in regards to the asset itself, but with respect to comparative change in other assets as well. 2. conventional wisdom when the branches of government are divided, less change comes from the government, so you can expect that your expectations for the future will remain stable, and that confidence reduces risk aversion, resulting in higher pricing.
Therefore, the market going down cannot be attributed purely to a question of who is in charge, because the democrats can't pass anything unless they get a nice number of Republicans on their (possible I suppose, but I don't think anyone is expecting that right now). But it could be attributed to expectations of what Republicans would do vs now that they can't.
Besides, it's basically impossible (or at least for layman, who knows if you're a market maker?) to accurately attribute a day's market behavior to any event, with a compelling causal explanation (sometimes your attribution could also be explained to have the opposite effect than the one you're trying to explain) and not ignoring alternate possible explanations.
It's kinda annoying when even otherwise serious people invoke the market movement of a day as an meaningful event that can be explained (notwithstanding that by tomorrow the effect might have reversed). But unfortunately business news like CNBC promotes that kind of thinking.