Georgia Senator Johnny Isakson has announced that he will be leaving the Senate at the end of 2019, setting up the Peach State as a Senate battleground for the 2020 election:

Sen. Johnny Isakson (R-Ga.) said Wednesday that he will resign at the end of 2019, citing health problems.

Isakson, who was reelected to a third term in 2016, said in a statement that he has informed Georgia Gov. Brian Kemp (R) of his decision, effective Dec. 31.

“I am leaving a job I love because my health challenges are taking their toll on me, my family and my staff. My Parkinson’s has been progressing, and I am continuing physical therapy to recover from a fall in July. In addition, this week I had surgery to remove a growth on my kidney,” the senator said.

Isakson was hospitalized last month with fractured ribs after a fall at his Washington apartment.

In a statement issued after his release, Isakson’s office said that symptoms of his Parkinson’s disease could lengthen the recovery process. Isakson revealed in 2015 that he had received a diagnosis of Parkinson’s two years earlier, saying that he experienced stiffness in his left arm and a slower gait as a result of the condition.

Isakson is chairman of the Senate Committee on Veterans Affairs and the Senate Select Committee on Ethics.

Isakson was set to serve through the 2022 election. Under Georgia law, according to the National Conference of State Legislatures, Kemp will make an appointment to replace Isakson pending a special election to be held concurrently with the 2020 general election.

More from the Atlanta Journal-Constitution:

U.S. Sen. Johnny Isakson said Wednesday he was stepping down from office at the end of 2019 as he struggles with Parkinson’s disease, setting up two elections for U.S. Senate in Georgia in 2020. 

Isakson, a three-term Republican, said he decided to step down because of the “mounting health challenges” that includes several falls from Parkinson’s disease and surgery this week to remove a growth on his kidney.

“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,” said Isakson, 74, in a statement.

Gov. Brian Kemp, a Republican, will appoint a replacement for Isakson, who was elected to a third term by a wide margin. Though his term doesn’t expire until 2022, Isakson’s retirement means the seat will be on the ballot next year.

Three Georgia Democrats have already announced a challenge to U.S. Sen. David Perdue, a first-term Republican who is up for election in 2020. Isakson’s seat will likely draw several other Democrats, who see Georgia as increasingly competitive. 

It’s not yet clear who Kemp will appoint to Isakson’s seat, though potential candidates include Attorney General Chris Carr, Georgia Senate Pro Tem Butch Miller and U.S. Rep. Doug Collins. 

A four-decade veteran of Georgia politics, Isakson is famed for his work ethic and busy schedule. His aides and allies long dismissed talk that he could step down early, and even floated the idea of a fourth term at the state GOP convention.

But he has grappled recently with complications with Parkinson’s, which limits his balance and mobility. He recently spent six days in an inpatient rehabilitation program after being hospitalized in Washington on July 16. And his statement Wednesday disclosed for the first time his kidney surgery.

“I look forward to returning to Washington on September 9 when the Senate goes back into session,” said Isakson.

“And after December 31, I look forward to continuing to help the people of Georgia in any way I can and also helping those who are working toward a cure for Parkinson’s.”

As noted, Isakson’s retirement sets up a unique, although not unprecedented, situation in Georgia that could play a role in determining which party controls the Senate when the new Congress convenes in January 2021. In addition to the re-election bid for Senator David Perdue, who was first elected to office in 2014, there will now also be a Special Election to fill the remainder of Isakson’s term, which expires after the 2022 election. In between then and the end of 2019, the seat will be filled by someone appointed by Republican Governor Brian Kemp. Most likely, of course, that appointment will be a fellow Republican.

There are already a number of Democrats running for their party’s nomination to face off against Perdue and it’s possible one or more of those could decide to jump into the other race instead. Alternatively, a contest for another Senate opening could cause other Democrats and Republicans to consider getting into the race. Inevitably, speculation on that end will likely focus most immediately on Stacey Abrams, the former Minority Leader of the Georgia House of Representatives who narrowly lost the Governor’s race last year. In the time since that loss, Abrams has been mentioned as a potential candidate for a Presidential running mate in 2020 and, briefly at least, as someone who may have entered the race for the Democratic nomination for President herself. However, less than an hour after Isakson’s announcement Abrams issued a statement saying she does not want to run for Senate.

At the very least, Isakson’s retirement has the potential to significantly impact the battle for Senate control. Currently, the GOP holds a 53-47 seat advantage in the Senate. This means that depending on whether or not the Trump/Pence ticket is re-elected, Democrats would need a net gain of 3-4 seats to take control of the Senate. The current math makes the Democrats’ task more difficult than one might think.

There are two Republican-held seats that are considered to be vulnerable. One is in Arizona, where Martha McSally will have to defend the seat she was appointed to earlier this year for the right to serve for the remainder of John McCain’s term. McSally is being challenged primarily by Mark Kelly, the former astronaut and husband of Gabby Giffords who seems likely to win the Democratic nomination and recent polling has Kelly pulling ahead of McSally for the first time. The other seat is in Colorado, where Cory Gardner must defend his seat in a state that went for Hillary Clinton in 2016. That task became more difficult last week when John Hickenlooper, the popular former Governor who had recently dropped out of the Democratic Presidential race, entered the race for the Democratic nomination to challenge Gardner. Assuming Hickenlooper wins that race, which seems likely, Gardner will face an uphill battle to stay in office.

Additionally, there are now two open-seat elections, one being the Special Election for Isakson’s seat and the other in Tennessee, where Lamar Alexander has decided to retire at the end of his current term. The seat in Georgia could be competitive, but Tennessee is likely to favor Republicans given the fact that the state will most likely be a safely red seat in the Presidential race next year.

On the other side of the balance sheet is Alabama, where Democrat Doug Jones faces the daunting prospect of defending his seat in a state that went overwhelmingly for Donald Trump in 2016 and will likely do so again in 2020. The outcome in these five races could decide who controls the Senate in 2021 and beyond