WASHINGTON — Senator Johnny Isakson, Republican of Georgia, announced on Wednesday that he would resign from his seat at the end of the year, citing health reasons for the decision.
His retirement sets the stage for two potentially competitive Senate races in Georgia, a state that Democrats have increasingly targeted, during a presidential election year. Mr. Isakson’s colleague, Senator David Perdue, is also a Republican and up for re-election. Republicans have held both of Georgia’s Senate seats since Mr. Isakson took office in 2005.
Mr. Isakson cited the toll from a number of health complications, including the progression of his Parkinson’s disease, a fall last month and surgery this week to remove a growth on his kidney, for “leaving a job I love” before the end of his term.
“It goes against every fiber of my being to leave in the middle of my Senate term,” Mr. Isakson said in a statement on Wednesday, adding that his “health challenges are taking their toll on me, my family and my staff.”
“But I know it’s the right thing to do on behalf of my state,” Mr. Isakson added.
Mr. Isakson, who won his third term in 2016, said he had already informed Georgia’s governor, Brian Kemp, of his decision to resign on Dec. 31. He missed the last few votes before the August recess after falling in his Washington apartment and fracturing four ribs. Mr. Kemp is expected to appoint Mr. Isakson’s replacement.
Mr. Isakson said he intended to return to the Senate in September and continue advocating for Georgia and those working toward finding a cure for Parkinson’s disease. He announced in 2015 that he had received a diagnosis of Parkinson’s disease two years earlier, after visiting a neurologist about stiffness in his left arm.
“Johnny’s deep and abiding love for Georgia and this nation have animated his years of service, making him not only a first-rate legislator, but also a man of the highest integrity,” said Senator Mitch McConnell of Kentucky, the majority leader. “His humor, humility and enduring faith have made him a role model to all of us who have had the pleasure to work with him.”
Mr. Isakson is widely respected for his bipartisanship and work ethic over decades of work in Georgia and national politics, and currently chairs both the Senate Select Committee on Ethics and the Veterans’ Affairs Committee. Multiple lawmakers lamented the loss of his bipartisan barbecues where, as Senator Richard J. Durbin, Democrat of Illinois, put it, “our differences disappeared slathered in sauce.”
Mr. Isakson, who touts being the only Georgian to have represented both the House and Senate in both the state and national legislatures, is also seen as a central figure in the Republican Party’s political might in the state.
“When Johnny Isakson speaks, people listen,” Mr. Perdue said in a statement. “He is a true statesman, and it shows every day. It is my high honor to serve as Johnny’s partner in the United States Senate. Like so many Georgians, I was surprised and saddened to learn that he will not finish the rest of his term. Together, we are able to get real results for Georgia.”
Senator Chuck Schumer of New York, the Democratic leader, said that “one of the many fine adjectives to describe Johnny Isakson is a word not used enough in the halls of Congress these days: kind.”
“Not only is Johnny a diligent and successful legislator, he is one of the kindest, most thoughtful senators,” he said in a statement. “Independent of any party or politics, everyone will miss Johnny.”
Mr. Isakson first entered politics in 1974, after two decades as a businessman in Georgia. Having served in the Georgia Air National Guard, he has advocated veterans legislation, including a multibillion-dollar overhaul of the veterans health care system. Mr. Isakson was a dominant voice in efforts to resolve the partisan tussle over allocating disaster funding toward several natural disasters in 2018.
Mr. Isakson is the fifth senator to announce retirement ahead of the 2020 elections. Three other Republicans, Senators Michael B. Enzi of Wyoming, Pat Roberts of Kansas and Lamar Alexander of Tennessee, and one Democrat, Senator Tom Udall of New Mexico, have all announced their intentions to step down.
A party official said that the person appointed to Mr. Isakson’s seat would have to compete in a primary before a special election in 2020, meaning that both Georgia Senate seats will be on the same ballot.
A spokesman for Stacey Abrams quickly tamped down speculation that the failed candidate for governor and rising Democratic star, who previously passed on challenging Mr. Perdue, would run for the seat. “While she will not be a candidate herself, she is committed to helping Democratic candidates win both Senate races next year,” said Seth Bringman, her spokesman.
In a statement on Twitter, the Georgia Democratic Party said that with Mr. Isakson’s retirement, “it has never been clearer: the path to victory runs through Georgia.”
Among the potential Republican contenders for Mr. Isakson’s seat are the state attorney general, Chris Carr, and lieutenant governor, Geoff Duncan, as well as members of Georgia’s delegation in the House. Representatives Doug Collins, the top Republican on the Judiciary Committee, and Tom Graves, a member of the Appropriations Committee, are both seen as possible successors to the seat.