Bernie Sanders. Someone had to say it. Just automatically, corporate tax rates go from 21 to 35%, so EPS for most companies fall, thus reducing the valuations of these companies. Forget margin expansion--companies will be too busy figuring out how to pay all of their staff $15/hour for the menial jobs there. High frequency traders and funds will go out of business because you now have to pay an insane amount to trade equities , thus reducing liquidity in the market and lowering valuation multiples.
Bernie Sanders is the biggest threat to your's and mine retirement account.