WASHINGTON (Reuters) - U.S. Senator Thad Cochran, chairman of the powerful Appropriations Committee and a member of the Republican Party’s old guard, said on Monday he will resign on April 1, putting another Republican Senate seat in play in elections this year.

FILE PHOTO: U.S. Senator Thad Cochran takes part in a news conference at the U.S. Embassy in Havana, Cuba, February 22, 2017. REUTERS/Alexandre Meneghini

“I regret my health has become an ongoing challenge,” Cochran, 80, first elected to the Senate in 1978, 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.”

Mississippi Republican Governor Phil Bryant has 10 days to appoint a replacement once Cochran formally notifies him of his retirement, the governor’s office said. A special election will follow in November to fill the remainder of Cochran’s term.

That means Mississippi will be electing two senators in November, when midterm elections are set for one-third of the Senate and all of the House of Representatives. The state’s other senator, Republican Roger Wicker, is running for re-election.

Republicans hold 51 Senate seats, including Cochran’s, and are trying to preserve congressional majorities; Democrats hope for an anti-Trump “wave” that would sweep them into power. Republicans are defending eight seats in the midterms; Cochran’s resignation means a ninth Republican-held Senate seat will be open. Democrats and independents that caucus with them are defending 26 seats.

Conservative Tea Party state Senator Chris McDaniel, who gave Cochran a scare in a bitterly fought Republican primary in Mississippi four years ago, announced last week that he would run against Wicker, charging that Wicker, like Cochran, was part of the Republican establishment.

But after Cochran’s announcement on Monday, McDaniel issued a statement suggesting he did not rule out withdrawing from that race and running for Cochran’s seat instead.

“All options remain on the table as we determine the best way to ensure that Mississippi elects conservatives to the United States Senate,” McDaniel said.

Cochran has been frequently absent from the Senate this year due to poor health. When first elected to the Senate in 1978, he became the first Republican in more than 100 years to win a statewide election in Mississippi. He had previously served three terms in the House of Representatives.

Cochran is the 10th-longest serving senator in U.S. history, his office said.

Lieutenant Governor Tate Reeves praised Cochran’s ability to steer money to Mississippi as Appropriations chairman. “He fought relentlessly for Mississippi, from helping our farmers get their crops to market to fighting for our citizens after Hurricane Katrina devastated much of the state,” Reeves said.

(Corrects number of seats Democrats and independents are defending to 26 from 25, paragraph 5)