Updated at 6:02 p.m. ET
Citing his ailing health, Sen. Thad Cochran, R-Miss., announced he will resign on April 1, setting up a special election this November.
"I regret my health has become an ongoing challenge," Cochran said in a statement Monday. "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."