In 2010, the Republicans SMASHED the Democrats in House majority. What happened to those voting populations? Why is there such wide swings? Do people really change their minds that much?
Ballot harvesting in CA flipped a bunch of seats.
I think in off-year elections it is common for the party that just won the White House to get complacent - so the Democrats experienced low turnout in 2010.
Meanwhile, Republicans who are pissed off that Obama won and determined to defeat his agenda, are motivated to gain or keep control of Congress because that’s their only recourse.
That is the same reason 2018 was bad for Republicans.
Something like that.