Actually they’ve been pretty much like this since the end of the Civil War. There’s been a couple of outliers like Bush 1 being the first sitting VP to win the presidency in over 100 years, and of course FDR, offset Carter going 1 term. But for the most part the pattern runs pretty solidly, 1 side wins the presidency, loses seats in the midterm, wins re-election, loses even more seats in the second midterm, other side wins the presidency, lather rinse repeat. I read a great article (right here actually) on that pattern back in 2003, and that article predicted all the presidential and midterm results we’ve had since then. The hardest part the Dems have keeping the pattern is that the 2018 Senate class is the same one that was Obama’s 2nd term and Bush’s 2nd midterm, the GOP has so few seats up in 2018 it’s basically impossible for the Dems to gain seats in that chamber.
“...the GOP has so few seats up in 2018 it’s basically impossible for the Dems to gain seats in that chamber.” But very likely that they’ll lose a few.