Midterms historically precede a strong year for the S&P 500. What to know if Democrats win back Congress. Investors are anxious about the 2026 midterms. Here's what historical S&P 500 data shows happened before and after past elections. Every midterm election since 1950 has set off the same chain reaction in the stock market, and with November approaching, Wall Street is quietly watching one of history's most stubborn.