Sen. Thad Cochran, R-Miss., citing poor health, announced Monday that he will resign April 1 from the Senate.

The 80-year-old Cochran, who is chairman of the Senate's Appropriations Committee, has been ill since at least last year with urological problems.

First elected to the Senate in 1978, Cochran recently has been frequently absent from that chamber, as has his GOP colleague, Sen. John McCain of Arizona, who is undergoing treatment for cancer.

Mississippi Gov. Phil Bryant, a Republican, will appoint someone to replace Cochran temporarily.

Bryant's pick would maintain the GOP's narrow, 51-seat majority in the 100-seat Senate — at least for now.

But a special election will be held in November to select a permanent replacement for the remainder of Cochran's term.

The state's other U.S. senator, Roger Wicker, is running for re-election this year. Wicker, who is also a Republican, faces a primary challenge from Chris McDaniel.

Mississippi is a solidly Republican state, giving the party a decided edge in holding on to both Senate seats. But the GOP's loss of a Senate seat from Alabama in a special election in December won by Democrat Doug Jones underscores the fact that nothing is certain in politics.