Inflation. When inflation is high, people want change. The democrats won because voters rejected inflation. (Which is the same reason Trump won in 2024, BTW, inflation was high, so voters wanted change). Anyway, if the democrats want to keep power in 2028, they have to deal with inflation, and deal with it hard and fast. That means (much) higher interest rates, lower government spending, and, possibly, tax increases. All of these make stocks go down.