The battle for the Senate is heating up as Republicans look to win back the majority in next year's midterm elections.

Several GOP senators have already said they won't seek reelection in 2022, with Sen. Roy Blunt (Mo.) becoming the latest with his announcement Monday.

The retirements are likely to pose some challenges for Republicans as Democrats eye pick-up opportunities in battleground states.

Here are the Senate Republicans who have announced they won't run again.

Sen. Roy Blunt (Mo.)

Blunt announced Monday that he would not seek reelection.

“After 14 general election victories — three to county office, seven to the United States House of Representatives, and four statewide elections — I won’t be a candidate for reelection to the United States Senate next year,” he said.

Sen. Richard Burr (N.C.)

Burr said when he was running for reelection in 2016 that his next term would be his last in the Senate.

"We can showcase all the work we have done for North Carolina families in the United States Senate in stark contrast with Deborah Ross who defended countless radical and out of touch ACLU policies," Burr said in 2016, referring to his Democratic opponent at the time.

Sen. Rob Portman (Ohio)

Portman announced Jan. 25 that he would not seek a third term as senator.

“I decided to make my announcement now because I have made up my mind. Over the next two years, I look forward to being able to focus all my energy on legislation and the challenges our country faces rather than on fundraising and campaigning,” he said.

Sen. Richard Shelby (Ala.)

Shelby, who was first elected to the Senate as a Democrat in 1986, said Feb. 8 that he would not run for reelection in 2022.

“Although I plan to retire, I am not leaving today," he said. "I have two good years remaining to continue my work in Washington. I have the vision and the energy to give it my all.”

Sen. Pat Toomey (Pa.)

Toomey announced Oct. 5 that he will retire from the Senate after the 2022 midterm elections. He also ruled out a run for governor of Pennsylvania.

“I will not be running for reelection in 2022 and I will not be running for governor," he said. "I will serve out the remainder of my term for a little over two years that are left to the current term, and after that my plan is to go back to the private sector.”