As is well known, presidential election outcomes are somewhat predictable based on economic performance. Votes for the U.S. Congress, are to a large part determined by party balancing. Right now, the Republicans control the executive branch, both houses of congress, and the judiciary, so it makes sense that voters are going to swing toward the Democrats. Political scientists Bob Erikson, Joe Bafumi, and Chris Wlezien have written a lot about this; see for example here, here, and here.
Here’s the deal with party balancing. Most voters will go with their party almost all the time. But there’s some subset of swing voters, and they’ll make the difference in marginal seats.
This simple idea can cause a lot of confusion. For example, in 2010, when the swing was going the other way, we discussed a a news commenter who characterized preference for divided government as the product of irrationality and “unconscious bias.” His mistake was to think of the electorate as a single entity (“Americans distrust the GOP. So why are they voting for it?”) rather than to recognize that different voters have different preferences.
Ironically, one reason the Democrats may well not regain the Senate in 2018 is . . . party balancing in 2016! Most people thought Hillary Clinton would win the presidency, so lots of people voted Republican for congress to balance that.