Senator Thad Cochran (R-MS) announced on Monday that he will resign from the Senate on April 1, ending a four-decade congressional career and triggering a fall election that could carve new divisions in the Republican Party and put the GOP Senate majority at greater risk.

Cochran, 80, has been suffering from health problems in recent months. He missed several weeks in the Senate last fall while recuperating and has been keeping a low public profile since his return to the Senate.

“I regret my health has become an ongoing challenge,” Cochran said in a statement. “I intend to fulfill my responsibilities and commitments to the people of Mississippi and the Senate through the completion of the 2018 appropriations cycle, after which I will formally retire from the U.S. Senate.”

First elected to the Senate in 1978 after a stint in the House, Cochran, who is chairman of the Appropriations Committee, a powerful panel with jurisdiction over government spending,  is one of the longest-serving members of Congress in history.

When Cochran steps down, the chairmanship is expected to pass to Senator Richard C. Shelby (R-AL), who is next in the line of seniority. Cochran has not presided over a public hearing since early September. Senators and staffers are working behind closed doors to draw up the final details of the annual federal agency budgets, with a March 23 deadline approaching.

“He’s been a good friend of mine for 32 years, and so I wish him well,” Senator Shelby told reporters in the Capitol shortly after Cochran’s announcement. He confirmed that he would like to claim the gavel. Shelby, 83, went out of his way to vouch for his own health. “I’ve still got a lot of energy,” he said.

Beyond shaking up the Senate, Cochran’s exit will affect the battle for the Senate majority. It gives Republicans another seat to defend. Republicans hold a 51-to-49 advantage over Democrats, who are facing a tough map on which they are defending 10 seats in states President Trump won.

There will be two Senate races in Mississippi this year because of Cochran’s departure. A special election for his seat will be held on the same day as the regularly scheduled November 6 midterms. Meanwhile, Republican Governor Phil Bryant will be in charge of appointing a replacement for Senator Cochran.

Senate Majority Leader Mitch McConnell (R-Ky) had asked Bryant to consider appointing himself to the seat, according to people familiar with their conversations. However, on Tuesday Governor Bryant  announced that he is taking himself out of the mix for the U.S. Senate seat being vacated by Cochran, saying he will find another strong contender to keep the seat firmly in GOP hands.

State Senator Chris McDaniel, who previously announced a primary challenge to Republican Senator Roger Wicker, has indicated he may switch races to take on Governor Bryant’s pick, but has not announced his decision.

Meanwhile, Mike Espy, former U.S. Representative and Secretary of the U.S. Department of Agriculture confirmed this week that he intends to run for the U.S. Senate seat vacated by Cochran.  Espy was elected as a Democrat to the 100th Congress in 1986 from Mississippi’s 2nd congressional district.  He became the first African American to represent Mississippi at the Federal level since Reconstruction.  He was reelected three times.  Just days after taking office for his fourth term, he resigned after being appointed as Secretary of Agriculture under President Clinton.

Unlike the regular election this year for Republican Senator Roger Wicker’s (R-MS) seat, candidates for Cochran’s seat would not compete in a primary, and if no one got more than 50 percent of the vote in November, the top two finishers would compete in a runoff.