By BEN NADLER and RUSS BYNUM, Associated Press

ATLANTA (AP) — Republican Sen. Johnny Isakson's announcement Wednesday that he will retire at year's end because of health problems instantly raised Georgia's status as a must-watch 2020 battleground by ensuring both of its Senate seats will be on the ballot.

Isakson's pending departure after more than four decades in Georgia politics gives Democrats a surprise opening in a Southern state where the GOP's lock on statewide elections has shown signs of slipping. The state's junior senator, Republican David Perdue, is already a top target as he seeks reelection for the first time next year.

Isakson, 74, said he is leaving the job he loves because "mounting" health issues, including Parkinson's disease, are "taking their toll." He won a third term in 2016 and would have faced reelection in 2022.

"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 in a statement . "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."

The November 2020 ballot in Georgia will now include a special election for the remaining two years of Isakson's term in addition to Perdue's race for another six-year term.

It's a doubly tantalizing target for Democrats who increasingly believe that Georgia, which has become less rural and less white in recent decades, stands on the verge of becoming a swing state after roughly two decades of leaning solidly Republican.

"Georgia is going to be a major decision maker in the reelection of Donald Trump and control of the U.S. Senate now," said Brian Robinson, a Republican political adviser in Georgia who served as communications director under former Gov. Nathan Deal.

It's an uphill battle for Georgia Democrats, who haven't elected a governor or U.S. senator since 1998. Bill Clinton was the last Democratic presidential candidate to win here, in 1992.

But statewide races have become more competitive in recent elections. Trump won Georgia by just over 5 percentage points in 2016, compared with his double-digit victory margins in other Deep South states. In last fall's race for Georgia governor, Republican Brian Kemp beat Democrat Stacey Abrams by just over 1% of the vote.

"This is yet another seat Republicans will need to defend next year in an increasingly competitive battleground," said Stewart Boss, a spokesman for the Democratic Senatorial Campaign Committee.

Kemp gets to appoint Isakson's replacement, who will serve until the special election. Isakson plans to step down in December.

There will be no primary elections for seat, meaning the November 2020 contest will be open to qualified candidates from all political parties. That could significantly increase the likelihood of a runoff, required by Georgia law if no candidate receives over 50% of votes.

Former White House official Nick Ayers, who recently returned to Georgia, said in an email that he won't offer himself for the seat. Republican strategists say other potential candidates include Georgia Reps. Doug Collins and Tom Graves, and statewide officers such as Lt. Gov. Geoff Duncan and Attorney General Chris Carr.

"This is a very short sprint to the 2020 election," Robinson said. "You're going to need somebody who's a proven fundraiser, has a good built-in network and already has some knowledge of the issues. That narrows the field somewhat."

Three Democrats have already declared their candidacy for Perdue's seat.

Abrams, who had been wooed to run against Perdue, "will not be a candidate" for Isakson's seat, a spokesman said on Twitter. She will instead continue to focus on voter access issues in Georgia and nationally.

As chairman of the Senate Veterans Affairs Committee, Isakson has worked to expand programs offering more private health care choices for veterans. He said he plans to return to Washington when the Senate resumes next month. But deteriorating health will keep him from staying long.

Isakson was diagnosed with Parkinson's in 2013. The chronic, progressive movement disorder often left him walking the Capitol's corridors with a noticeably slower, shuffling gate. After winning reelection in 2016, Isakson underwent surgery the following year to address spinal deterioration. At times, he has been in a wheelchair.

He's been recovering from a July fall at his Washington apartment that fractured four ribs. And Isakson revealed Wednesday that earlier this week he had surgery to remove a growth from one of his kidneys.

Fellow senators from both parties saluted Isakson's tenure. Sen. Lindsey Graham, R-S.C., called Isakson "the gold standard" for the Senate. Sen. Dick Durbin of Illinois, the Democratic whip, said Isakson is "one of our best."

Isakson, who made millions of dollars in real estate, first won election to the state legislature in 1976. His years as a Republican lawmaker when Democrats still dominated Georgia politics shaped him into an affable consensus builder — a style he clung to even as Congress became bitterly partisan, said former U.S. Rep. Jack Kingston, a Georgia Republican who worked with Isakson at both state and federal levels.

"There are a lot of times when you want to tell the other side to go to hell," Kingston said. "Johnny always refrained from that. But he never was intimidated by those in power, whether it was Nancy Pelosi or Donald Trump."

___

Bynum reported from Savannah, Georgia. AP Chief Congressional Correspondent Lisa Mascaro contributed from Washington.