The chart above is from the Gallup Poll. It shows the feeling among the public at this time of the year regarding off-year elections for Congress from 1994 to 2018. The latest survey was done between April 9th and 15th of a random national sample of 1,510 adults, with a 3 point margin of error.

Note that there is currently a fairly strong anti-incumbent sentiment. About 26% say most members of Congress deserve to be re-elected, and only 51% say their own member of Congress deserves to be re-elected. Those numbers should worry Republicans the most, since they have the largest number of incumbents (and other polls have shown they are being blamed for congressional dysfunction).

We could easily be looking at another year like 2010, when control of Congress flipped. In fact, the 2010 numbers looked slightly better for incumbents that 2018 numbers do.