All they need is a gain of three House seats. Going back even further, since World War II the out party has gained an average of more than two dozen House seats. AdvertisementIn Trump’s last midterm election, in 2018, Democrats won 40 House seats — including seven in California — to seize control. There are 435 House seats on the ballot next year. Over the past three decades, every time the president’s net job approval was negative a year from the midterm election, his party lost House seats.