Mississippi Sen. Thad Cochran, a Republican who has represented the state since 1978, has been in ill health for a while, and state and national Republicans have been speculating for months that he'd resign this year. On Monday, Cochran's office announced he would indeed step down on April 1. GOP Gov. Phil Bryant will appoint a successor, and that person will serve until a special election can be held.

Mississippi has an unusual special election law. All the candidates will run on one officially nonpartisan ballot on Nov. 6, and if no one takes a majority, the two candidates with the most votes advance to a runoff at a later date. By contrast, in normal state elections, both parties hold primaries, require a primary runoff if no one takes a majority, and the nominees compete in the general election with their party affiliation listed.

Mississippi is a very red state, but GOP leaders have fretted that state Sen. Chris McDaniel, a tea partier who almost beat Cochran in 2014, could emerge as their nominee and cost them the race. Donald Trump and Senate Majority Leader Mitch McConnell have been talking to Bryant about whom he might appoint to the Senate to hold off McDaniel in a special, and they've even encouraged the governor to pick himself; Bryant reportedly isn't interested.

For his part, McDaniel announced at the end of February that he would challenge Sen. Roger Wicker, who is running for the normally-scheduled six-year term. However, McDaniel didn't rule out switching races and running in a special if there was one, and now he'll have his chance. Democrats don't have a great bench in Mississippi, but they'll want to field a viable candidate in case the GOP picks a weak nominee like they did in neighboring Alabama.