Sen. Richard Shelby, the Republican senior senator from Alabama and the leading Republican on the Senate Appropriations Committee, announced his decision not to seek re-election in 2022 and his retirement from politics Monday.

Shelby, 86, has served six terms in the United States Senate, first elected in 1986 after serving four terms in the U.S. House representing Alabama's 7th Congressional District and eight years in the Alabama state Legislature.

In a statement Monday, he said serving in the Senate has "been the opportunity of a lifetime."

"I am grateful to the people of Alabama who have put their trust in me for more than forty years. I have been fortunate to serve in the U.S. Senate longer than any other Alabamian. During my time in the Senate, I have been given great opportunity, having chaired four committees: Appropriations, Rules, Banking, and Intelligence. In these positions of leadership, I have strived to influence legislation that will have a lasting impact – creating the conditions for growth and opportunity," Shelby said.

"I have done my best to address challenges and find ways to improve the day-to-day lives of all Americans," he continued, noting his support for policies to expand access to education and improve the quality of Alabama schools, his work to increase Alabama's role in space exploration, and his support for using Alabama's river systems and the Port of Mobile to benefit the nation.

"My service in the U.S. Senate would not have been possible without those who have encouraged me over the years. I am particularly grateful for the support of my wife, Annette, and my entire family," Shelby said, also thanking his staff.

Though Shelby will not seek re-election in 2022, he said he intends to serve his remaining two years in the Senate until his current term expires.

Speculation has already started about who could run in the Republican primary to succeed Shelby. Rep. Mo Brooks (R-Ala.), who unsuccessfully ran for Senate in 2017, told Roll Call, "I am running for election in 2022, either for my House seat or for the Alabama Senate race."

Brooks, a staunch Trump supporter, led the GOP effort in the House to challenge the Electoral College votes from states whose results were disputed by the Trump campaign. In response, Roll Call reports House Democrats have introduced a resolution to censure Brooks for that effort and for comments he made at the rally on Jan. 6 before a mob stormed the Capitol building. Brooks believes the attacks from Democrats have contributed to his popularity among Alabama Republicans.

"Quite frankly, the last three months of scurrilous and palpably false attacks on me by Socialist Democrats and their Fake News Media allies have been a wonderful blessing in disguise because they have sent my statewide name I.D. and Republican primary support through the roof," Brooks wrote.

Another potential top-tier candidate for Senate may be Business Council of Alabama CEO Katie Boyd Britt, formerly Shelby's chief of staff, who spoke to Roll Call, according to GOP strategists, and would likely enter the race with Shelby's backing.

Shelby is the fourth Republican senator to announce his retirement from the Senate, following Sens. Richard Burr (R-N.C.), Rob Portman (R-Ohio), and Patrick Toomey (R-Pa.)