Sen. Johnny Isakson, R-Ga., will resign at the end of the year as he faces various health problems.
In a statement Wednesday, the 74-year-old lawmaker said his Parkinson's disease has advanced. In addition, he has undergone physical therapy following a fall in July and had a procedure to remove a growth on his kidney this week.
"With the mounting health challenges I am facing, I have concluded that I will not be able to do the job over the long term in the manner the citizens of Georgia deserve," Isakson said. "It goes against every fiber of my being to leave in the middle of my Senate term, but I know it's the right thing to do on behalf of my state."
Isakson has represented Georgia in the Senate since 2005. His term ends in January 2023.
The senator's departure creates another potentially competitive race as Republicans and Democrats fight for control of the chamber — and the next president's legislative agenda — next year. Democrats have already targeted incumbent Georgia Republican Sen. David Perdue, who faces reelection in 2020.
Georgia's Republican governor, Brian Kemp, will appoint a successor, then the state will hold an election for the Senate seat on the same day as the 2020 general election, said Tess Hammock, a spokeswoman for the Georgia secretary of state's office.
A special election for his seat would draw intense national interest as Republicans try to defend their 53-47 majority in the Senate.
Democrats have flooded into the race for Perdue's seat following a closer-than-expected gubernatorial race last year. Kemp beat former state House Minority Leader Stacey Abrams by about 1.5 percentage points, giving Democrats hopes of making Georgia into a Senate or even presidential battleground in 2020.
Democrats who hope to challenge Perdue include former lieutenant governor candidate Sarah Riggs Amico and former Columbus Mayor Teresa Tomlinson.
In a statement Wednesday, National Republican Senatorial Committee Chairman Sen. Todd Young, R-Ind., called Isakson a "steadfast conservative leader." The senator said Isakson "will be missed, but we look forward to the men and women of Georgia electing another strong Republican leader in 2020 alongside David Perdue."
Abrams declined to jump into the Democratic primary despite prodding from national party leaders. She now could face pressure to run for Isakson's seat, as well.
Abrams quickly shot down speculation about a Senate run.
"Leader Abrams' focus will not change: she will lead voter protection efforts in key states across the country, and make sure Democrats are successful in Georgia in 2020," a spokesman for the former Georgia lawmaker said. "While she will not be a candidate herself, she is committed to helping Democratic candidates win both Senate races next year.