Stacey Abrams on Wednesday ruled out a bid for the U.S. Senate seat being vacated by Sen. Johnny Isakson, hours after the Georgia Republican announced he is resigning at the end of the year due to health problems.

Isakson, who revealed in 2015 that he has Parkinson’s disease, said in a statement Wednesday that the disease "has been progressing." He also said that he had surgery this week to remove a growth on his kidney — a renal cell carcinoma — and that he had suffered four fractured ribs and a torn rotator cuff in a fall last month.

“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, the chairman of the Committee on Veterans' Affairs, 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, 74, was elected to his third term in the Senate in 2016 and would not have been up for re-election until 2022.

His sudden departure gives Democrats a crucial opportunity to flip a seat and possibly control of the Senate — but Abrams, the state's 2018 Democratic nominee for governor and likeliest target of the party's recruiting efforts, quickly took herself out of contention. Abrams. the former minority leader of the Georgia House of Representatives, had also ruled out challenging GOP Sen. David Perdue, who is up for re-election next year.

“Our thoughts are with Senator Isakson and his family," her spokesman, Seth Bringman, said. "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."

"While she will not be a candidate herself," Bringman added, "she is committed to helping Democratic candidates win both Senate races next year.”

Abrams has also said she will not run for president. Many suspect she is waiting to run for governor in 2022 against Brian Kemp, the Republican who defeated her last year, or leaving herself open for a vice-presidential appointment.

Georgia was already expected to be a key battleground for the White House and Senate since Perdue is up for re-election. But now the state will have an unusual "double-barreled" Senate contest, with both of its seats up for grabs at the same time.

There were two such elections last year in less competitive states, Minnesota and Mississippi, but historically they are rare.

Isakson won re-election in 2016, 55 percent to 41 percent, but Abrams narrowly lost to Kemp. Many Democrats viewed that as proof that the state's growing minority population and a shift in the suburbs away from the GOP under President Donald Trump have made Georgia competitive.

Lucy McBath, an African American gun control activist, last year won a Republican-leaning congressional seat in Atlanta's wealthy northern suburbs that had been in GOP hands for decades and was once represented by Newt Gingrich.

"We have a chance to deliver the U.S. Senate and a new president," Nikema Williams, a Georgia state senator and chairwoman of the state Democratic Party, told NBC News. "I expect Georgia to move to the top of every list."

"In 2018, we came closer than we ever have," Williams added. "We are in play at every level."

The situation was fluid Wednesday as news of Isakson's retirement scrambled the Georgia political landscape, with calls and text messages flying among activists and operatives to suss out who would run in both races.

Georgia Democrats say other names in the mix to run for Isakson's seat include: Raphael Warnock, the reverend of Ebenezer Baptist Church in Atlanta, where Martin Luther King Jr. preached; Jon Ossoff, who raised tens of millions of dollars in his unsuccessful but high-profile congressional special election in 2017; former Senate candidate Michelle Nunn, who now runs an international humanitarian group; former state Sen. Jason Carter, the grandson of former President Jimmy Carter; Stacey Evans, who lost the primary race for governor last year to Abrams; State Sen. Jen Jordan; and DeKalb County District Attorney Sherry Boston, among others.

Republicans, meanwhile, said they're confident they can defend both Senate seats in Georgia.

"Johnny Isakson has been a steadfast conservative leader who has served Georgians with the highest integrity and distinction in the U.S. Senate," Sen. Todd Young, R-Ind., the chairman of the National Republican Senate Committee, said in a statement. "He will be missed, but we look forward to the men and women of Georgia electing another strong Republican leader in 2020 alongside David Perdue."

Under Georgia election rules, Kemp will appoint a replacement for Isakson, who will hold the seat until a special election occurs next November. The winner of the special election will serve the remainder of Isakson's term — through 2022 — and would have to run for re-election then for a full term.

According to a Republican source familiar with the discussions, Kemp could be considering Georgia Attorney General, and Isakson's former chief of staff, Chris Carr; Lt. Gov Geoff Duncan; U.S. Rep. Doug Collins, the Republican ranking member on the House Judiciary Committee; and U.S. Rep. Tom Graves.

Kemp said in a statement Wednesday that he would appoint Isakson's replacement "at the appropriate time."

Meanwhile, Isakson's colleagues in both parties lamented his departure and praised him for his years of service in the Senate.

Senate Majority Leader Mitch McConnell, R-Ky., called Isakson a "first-rate legislator," "a man of the highest integrity" and a "legislative workhorse."

Senate Minority Leader Chuck Schumer, D-N.Y., for his part, called Isakson a "diligent and successful legislator" and "one of the kindest, most thoughtful senators."

"Independent of any party or politics, everyone will miss Johnny," Schumer said.

Perdue, Isakson's fellow Georgian, called his colleague a "true statesman" who "has always been a champion for the people of Georgia, especially our veterans."

CORRECTION (Aug. 28, 2019, 2:40 p.m. ET): A previous version of this article misstated the year Sen. Johnny Isakson publicly revealed that he had Parkinson's disease. It was 2015, not 2013 (the year he was diagnosed).