» Photos: Johnny Isakson through the years

» Related: Georgia's most enduring Republican sets an end date to a 45-year career

» Timeline: Key moments in Johnny Isakson's public life

» Related: How will U.S. Sen. Johnny Isakson's replacement be chosen?

Isakson's announcement, which stunned Republican insiders, sets up an epic slate of contests in 2020. U.S. Sen. David Perdue already faces three challengers in his bid for a second term. Metro Atlanta's suburbs are home to two of the nation's most competitive U.S. House races, and control of the Georgia House is in play.

And Democrats — emboldened by last year's tight race for governor — are pushing to flip Georgia in the presidential race for the first time since 1992, when Bill Clinton carried the state.

“First and foremost, Georgia needs to say a long and thoughtful thank you to Johnny Isakson,” said John Watson, a former Georgia GOP chairman. “But as we witnessed last election cycle, when we had unprecedented money and attention, we ain’t seen nothing yet.”

He added, “This puts Georgia front and center on every political map, and the times will only get more interesting.”

Democrats are quick to agree. State Sen. Nikema Williams, the chairwoman of the state Democratic Party, predicted that Isakson’s retirement will clinch Georgia’s status as a swing state.

“We are the battleground state, and Georgia Democrats are ready to fight and deliver both the Senate and the presidency for Democrats across the country in 2020,” she said.

Johnny Isakson kicks off his campaign for Senate in 2003. (Kimberly Smith/staff) Credit: KIMBERLY SMITH Credit: KIMBERLY SMITH

‘Complicated chess game’

Kemp has several months to make his highest-profile decision since he took office earlier this year.

His aides indicated that the governor has no favorite for the coveted spot, but among the possibilities are a constellation of prominent officials including U.S. Agriculture Secretary Sonny Perdue, Georgia Attorney General Chris Carr and Lt. Gov. Geoff Duncan.

He's also likely to consider an outside-the-box pick, much as he did earlier this year when he picked John King, a Doraville police chief, to serve as insurance commissioner and become the state's first Hispanic constitutional officer. That could mean selecting a business executive such as Kelly Loeffler, who considered a U.S. Senate run in 2014.

“It’s a complicated chess game. It’s not just about one seat,” said Jay Morgan, a veteran operative and former executive director of the Georgia GOP. “You have to think about how it affects (David) Perdue’s race and, ultimately, about Kemp’s own re-election campaign in 2022.”

It also could have far-ranging national implications. Democrats already saw Georgia as a must-win state in order to flip control of the Senate, where Republicans have a 53-47 advantage. The second contest will just heighten the state’s importance.

It sent Democrats scrambling to reassess options. Perdue had already drawn three challengers, and all of them — car-hauling executive Sarah Riggs Amico, Clarkston Mayor Ted Terry and former Columbus Mayor Teresa Tomlinson — said they will stay in the race.

The format of the Isakson contest could be enticing because the special election will pit candidates from all political parties on the same November ballot instead of making them compete in a polarizing party primary to hash out the nomination.

That means a far higher turnout — after all, President Donald Trump will be at the top of the ticket — which could make it tempting for more moderate candidates who want to appeal to a broader base.

And it all but guarantees a runoff between the two top finishers on Jan. 5, 2021, a contest that could direct even more of the nation’s attention toward Georgia.

Among the potential Democratic contenders for the seat are the Rev. Raphael Warnock, the pastor of Ebenezer Baptist Church; Jon Ossoff, a former candidate for Georgia’s 6th Congressional District; Jason Carter, the runner-up for governor in 2014; and Michelle Nunn, who was defeated by Perdue in the 2014 Senate race.

The state's most prominent Democrat, however, ruled herself out. Stacey Abrams, who narrowly lost last year's race for governor and earlier ruled out a 2020 bid for Perdue's seat, said through a spokesman that she would not run next year. Instead, she will focus on an expansion of her voting rights initiative.

“While she will not be a candidate herself, she is committed to helping Democratic candidates win both Senate races next year,” said the spokesman, Seth Bringman.

May 9, 2014 Athens - U.S. Senator Johnny Isakson gives the graduation address during the Georgia 2014 Spring undergraduate commencement at Sanford Stadium in Athens on Friday night, May 9, 2014. HYOSUB SHIN / HSHIN@AJC.COM Credit: HYOSUB SHIN / AJC Credit: HYOSUB SHIN / AJC

‘Heart and soul’

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 they even floated the idea of a fourth term at the state GOP convention in May.

But he has grappled 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.

A successful real estate agent before he entered politics, Isakson ran for office in his 20s and quickly emerged as a rising star. Back then, Republicans were a rarity in state politics: By the time he took over as the Georgia House’s top Republican in 1983, there were only 24 GOP lawmakers in the 180-member Georgia House.

He helped build the Georgia GOP from his Cobb County base, but he has also gained bipartisan support by cutting deals with Democrats and offering candid criticism of President Donald Trump and other Republicans. He won re-election easily in 2016, capturing roughly 55% of the vote.

His retirement triggered praise from state leaders from both parties, including Atlanta Democratic Mayor Keisha Lance Bottoms, who said Isakson’s departure is a “great loss” for Georgia. And Kemp said the state was “immeasurably blessed” by his leadership.

“No one embodies the heart and soul of Georgia more than Johnny Isakson,” he said.

One of the more personal statements came from Carr, who was a former top aide to Isakson and is married to his current chief of staff. He called Isakson a “shining example of how to be successful professionally, politically and personally.”

“Whenever I am confronted with a tough decision, I often ask myself: What would Johnny do?” Carr said.

Isakson, the chairman of two U.S. Senate committees, said he will make the most of his last months in office. He said he will return to Washington on Sept. 9 when the Senate goes back into session, and he suggested his public service won’t be done when he steps down.

“After Dec. 31,” he said, “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.”

Stay on top of what’s happening in Georgia government and politics at www.ajc.com/politics.