There's a couple of differences. Trump's approval rating has been stuck historically low and doesn't change from the 35-44 range. Voter turnout in the midterms were much higher than in many years, suggesting a dissatisfaction with the Republicans and Trump. Obama won reelection with support from strong turnout from Blacks in key states. Duplicate that turnout in 2020 with a strong Democratic candidate, and the key swing states are back in Democratic hands.