Citing health issues, Sen. Johnny Isakson of Georgia is resigning at the end of 2019.  The Republican was elected to a 3rd term in 2016. 

After much prayer and consultation with my family and doctors, I have made the very tough decision to leave the U.S. Senate at the end of 2019. It has been the honor of a lifetime serving the state of Georgia. This decision pains me greatly but I know it is the right thing to do.

— Johnny Isakson (@SenatorIsakson) August 28, 2019

Republican Gov. Brian Kemp will appoint a replacement who will serve until a special election is held concurrent with the general election in November, 2020. The winner of that race will complete the final two years of Isakson's term. 

Sabato's Crystal Ball is starting this race with a Leans Republican rating, the same it has assigned to Georgia's other U.S. Senate seat - held by Republican David Purdue - which will also be contested in 2020. Of note - while these double-barrel elections are relatively infrequent, the same party usually wins both. The last split result was in South Carolina back in 1966.