Reacting Wednesday to the sudden retirement of his mentor, Sen. Johnny Isakson, Georgia Attorney General Chris Carr said, "Whenever I am confronted with a tough decision, I often ask myself, What would Johnny do?"

Carr said in a news release that Isakson is "a shining example of how to be successful professionally, politically and personally. I will be forever grateful that he was willing to take a chance on me in 2004 and that he has served as a mentor to me ever since."

That was the year that Carr left Alston & Bird and moved to Washington, D.C., to serve as Isakson's chief of staff. When Gov. Nathan Dean appointed Carr attorney general and gave him the oath of office in November 2016, Isakson was there in the Capitol Rotunda to introduce Carr. "You've got a good man there," the senator told a reporter.

Carr added Wednesday: "For over 40 years, Johnny has served our state as a state legislator, State School Board Chairman, a member of the U.S. House of Representatives, and now as a United States Senator. Our state and our nation are better because of Johnny's service. I wish my friend and mentor well as he crosses the finish line of a stellar career in public service."

Isakson broke the news Wednesday morning.

"After much prayer and consultation with my family and my doctors, I have made the very tough decision to leave the U.S. Senate at the end of this year. I have informed Georgia Governor Brian Kemp today that I will resign my Senate seat effective December 31, 2019," the senator said in a news release. "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."

Isakson, 74, was diagnosed with Parkinson's disease in 2013. In July, he fell in his D.C. apartment, suffering four fractured ribs and a torn rotator cuff. On Monday, Isakson underwent surgery at WellStar Kennestone Hospital in Marietta to remove a 2-centimeter renal cell carcinoma from one of his kidneys.

Isakson said he has "always put my constituents and my state of Georgia first," but, "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. 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's Senate term ends in 2022, and there will be three years left in the term when he vacates the seat in December.

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

In 2016, Isakson won reelection with 54% of the vote and became the first Republican in Georgia to be elected to a third term in the U.S. Senate. After more than three decades in the real estate business, Isakson became the only elected official in Georgia to serve in the Georgia House, the Georgia Senate, the U.S. House and the U.S. Senate. Isakson also is a veteran of the Georgia Air National Guard and a graduate of the University of Georgia.

In a polarized political atmosphere, Isakson was known for bipartisan cooperation, courtesy and warmth on the Senate floor.

"The loss of Johnny Isakson from public life will leave a void in Georgia which is beyond comprehension," attorney and Georgia House Speaker David Ralston (R-Blue Ridge) said in a news release. "While I respect his reasons, I feel a tremendous sense of personal sadness – Johnny is a mentor, role model and friend. Over a distinguished career in the Georgia House, Georgia State Senate, State Board of Education, U.S. House and U.S. Senate, Johnny Isakson demonstrated that civility and reasonableness are virtues that will never go out of style. My family and I will continue to pray for Johnny, Dianne and the Isakson family."

The governor issued a joint statement with his wife, Marty Kemp.

"Senator Isakson is a lifelong family friend, and we are deeply grateful for his dedicated service to our state and nation, including alongside my father in the Georgia General Assembly," Marty Kemp said. "Georgia is incredibly blessed to have had Johnny on our side for all these years. Our family is praying for Johnny and Dianne as they embark on this new journey."

Gov. Kemp said, "No one embodies the heart and soul of Georgia more than Johnny Isakson," adding that his "list of accomplishments on behalf of the state that he loves is long and revered, but what Georgia should be most thankful for is the high standard that Johnny held as a true gentleman, a fighter for his constituents, a trusted advocate for our nation's veterans, and one of the greatest statesmen to ever answer the call of service to our country."

The news release also included the applicable law for the circumstances, O.C.G.A. § 21-2-542: "Whenever a vacancy shall occur in the representation of this state in the Senate of the United States, such vacancy shall be filled for the unexpired term by the vote of the electors of the state at a special election to be held at the time of the next November statewide general election, occurring at least 40 days after the occurrence of such vacancy; and it shall be the duty of the Governor to issue his or her proclamation for such election. Until such time as the vacancy shall be filled by an election as provided in this Code section, the Governor may make a temporary appointment to fill such vacancy."

"There is no vacancy until Isakson's formal resignation on December 31, 2019. Under Ga. Const. Art. V, § II, Para. VIII and Ga. Code. Ann. § 21-2-542, the Governor will make a temporary appointment where such person will serve until a special election is held on November 3, 2020," Kemp said. "I will appoint Senator Isakson's replacement at the appropriate time."