A Smart Politics analysis finds that there have been at least two U.S. Senators who did not seek another term in each of the 52 election cycles since the passage of the 17th Amendment in 1913, but almost all of the 33 lawmakers up for reelection in 2018 have already confirmed they are running (excepting Republican Bob Corker of Tennessee and Democrats Heidi Heitkamp of North Dakota, Tom Carper of Delaware, and Ben Cardin of Maryland).

