Senator Johnny Isakson has been a US senator since 2005. Many people have accurately labeled him a conservative, but he ranks as one of the Senate's most moderate members. He has butted heads with President Trump more than once. This includes his defense of Democratic Senator Jon Tester and late Republican Senator John McCain.

Isakson and the president's disputes may soon come to an end, as will Isakson's service in the Senate.

He has resigned, effective at year's end

Isakson has tendered his resignation. His cited reasoning is his health. Politico reports that the move came as a surprise to many.

But Isakson's health has been an issue for him in recent years. He suffers from Parkinson's disease. Other struggles he's faced include two back surgeries and multiple fractured ribs.

Since 2015, Isakson has chaired both the Veterans Affairs and Ethics Committees of the Senate. As The Hill reports, Isakson is popular among sitting senators on both sides of the aisle. Democratic Senator Mark Warner called him "a gentleman who's spent his career looking for common ground," on Twitter.

Prior to entering politics, Isakson served in the Air National Guard. He later graduated from the University of Georgia. Eventually, Isakson became president of a major real estate company.

Isakson first won a seat in the Georgia House of Representatives in 1976 after losing in 1974.

He would be re-elected six times and rise to the post of minority leader. In 1990, he was the unsuccessful Republican nominee for governor. Two years later, he was elected to the Georgia Senate.

Isakson made his first attempt at the US Senate in 1996. He lost the Republican nomination to Guy Millner. Millner would lose in the general election to Georgia Secretary of State Max Cleland.

Later in the year, he was appointed to lead the state's board of education.

In 1999, he was elected to the US House of Representatives from Georgia's 6th District in a special election. He succeeded former House Speaker Newt Gingrich, who'd refused the seat despite winning re-election the previous year. Isakson would be re-elected twice.

He finally won the Republican nomination for the US Senate in 2004. Conservative Democratic incumbent Zell Miller chose not to run for re-election. Isakson would win that general election and two more afterward.

A special election is to be held next year

Republican Georgia Governor Brian Kemp is likely to appoint a replacement for Isakson. But a special election is still to be held next year. It will probably coincide with the national election in November. Georgia's other Senate seat is being contested at the same time, on its regular schedule. It's currently held by Republican David Perdue.

A number of high-profile Democrats have already been mentioned as possible candidates. The filmmaker and former Congressional candidate is Jon Ossoff.

US Representative Lucy McBath is another.

A couple of possible contenders come from prominent families. Michelle Nunn has been brought up. Nunn was the unsuccessful Democratic Senate candidate in 2014 and is the daughter of former US Senator Sam Nunn. Former State Senator Jason Carter is another. Carter was the Democratic nominee for governor in 2014. His grandfather is former US President Jimmy Carter.

Several Republicans have been listed as a possible appointment to Isakson's seat. At the state level, there's Lieutenant Governor Geoff Duncan and Attorney General Christopher M. Carr. And at the federal level, there are US Representatives Doug Collins and Tom Graves.