Sen. Richard Shelby, R-Ala., to retire, becoming fourth Republican to not seek reelection in 2022

Sen. Richard Shelby, R-Ala., announced Monday he will retire after six terms, becoming the fourth Senate Republican to announce they won't seek reelection in 2022, when the GOP will try to regain the majority in the chamber.

Shelby started his political career as Democrat, serving eight years in the Alabama state Senate. He won election to the U.S. House in 1978, representing the state's 7th Congressional District. Shelby was first elected to the Senate in 1986, and switched parties and became a Republican in 1994.

"Today I announce that I will not seek a seventh term in the United States Senate in 2022," Shelby announced. "For everything, there is a season."

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The senator emphasized that "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."

But he stressed that "I am not leaving today. I have two good years remaining to continue my work in Washington. I have the vision and the energy to give it my all."

During his nearly 35 years in the Senate, Shelby chaired a important and powerful committees including the Banking Committee, the Intelligence Committee, and most recently Appropriations, which he used to direct federal funding to numerous projects in Alabama.

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The Senate is split 50-50 between the two parties, but the Democrats control the chamber due to the tie-breaking vote of Vice President Kamala Harris. That means Republicans only have to flip one seat in 2022 to regain the majority they lost following the Democratic sweep in last month’s twin Senate runoff elections in Georgia.

But the GOP is defending 20 of the 34 seats up for grabs next year. The difficult map isn’t the only obstacle facing the Republicans. They're defending open seats in two crucial battleground states – in Pennsylvania with the announced retirement of GOP Sen. Pat Toomey and in North Carolina, where Republican Sen. Richard Burr isn’t running for reelection. And two-term GOP Sen. Rob Portman of Ohio announced last month that he’ll retire from the Senate rather than run for reelection in 2022.

There’s also a potential headache in Iowa, where 87-year-old GOP Sen. Chuck Grassley is staying mum so far on whether he will run for an eighth six-year term in the Senate. In the battleground state of Wisconsin, Republican Sen. Ron Johnson hasn’t said if he’ll run for a third term.

But Republicans see pickup opportunities in the key battlegrounds of Georgia and Arizona, where Democrats Raphael Warnock and Mark Kelly won special elections to serve two years in the Senate and are up for reelection in 2022. The GOP also sees possible pickups in Nevada and New Hampshire, where first-term Democratic Sens. Catherine Cortez Masto and Maggie Hassan are running for reelection.

While holding onto Shelby's seat in the deeply red state of Alabama shouldn't be a problem in 2022, the battle to replace him could spotlight the current fissures in the GOP between more traditional consevatives and those loyal to former President Trump.

Shelby was usually a reliable vote for Trump, but opposed the then president's backing of scandal-ridden GOP candidate Roy Moore in the 2017 special election to fill the seat vacated by Jeff Sessions’ confirmation as attorney general. Moore then lost to Democrat Doug Jones.

Jones was crushed in November by Republican Tommy Tuberville, the former Auburn University football coach, as he ran for a full term in the Senate.

Sessions unsucessfully ran last year to try and return to the Senate but was defeated by Tuberville, who enjoyed Trump's support.

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A source close to Sessions told Fox News "no" when asked if Sessions plans to run for the seat in 2022.

Fox News' Alex Pappas contributed to this report.