Longtime GOP Sen. Thad Cochran to retire in April, setting up special election in November

Cochran was first elected to the U.S. Senate in 1978.

"I regret my health has become an ongoing challenge. 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," Cochran wrote in a statement released Monday.

Mississippi governor Phil Byrant, a Republican, will have 10 days to appoint Cochran's replacement after he steps down, according to state law. A special election will then be held on Nov. 6, meaning that both of Mississippi's U.S. Senate seats will be up for election in 2018.

According to state law, Cochran's seat will be filled by a non-partisan special election this November in which candidates from all political parties will run. If no candidates receive 50 percent of the vote or more, a runoff election between the top two candidates will decide who ultimately fills out the remainder of Cochran's term.

In a statement released Monday, McDaniel said he will "continue to monitor developments regarding his replacement and the Special Election that will follow," but said he is currently focused on his primary challenge to Wicker.

McDaniel ran a bitter campaign against Cochran in 2014, hoping to unseat the longtime senator, but ultimately fell short in his bid.

ABC News' Mariam Khan and Nia Phillips contributed to this report.