Well my ship’s been split to splinters and it’s sinkin’ fast; I’m drownin’ in the poison, got no future, got no past
By 
On Monday afternoon, Sen. Thad Cochran, the first Republican in over a century to win a statewide race in Mississippi and the longest-serving member currently in Congress, announced he would resign from office on April 1. The news wasn’t unexpected — Cochran’s worsening health began stirring up rumors of resignation as early as last year — but it still throws a wrench into the 2018 midterm elections, and in particular, the U.S. Senate map.
That’s because Cochran’s seat wasn’t scheduled to be up for election until 2020, so we’re looking at another special Senate election in the Deep South. As you might recall, Democrats have had some success with those recently. Like Alabama, a Mississippi special election will be a steep uphill climb for Democrats, but like Alabama, the seat could fall into their hands under the right circumstances. Several things would need to go right for Democrats to snag Cochran’s seat — perhaps a bad Republican candidate and a bad Republican political environment — but the 2018 Senate map offers the party such slim pickings that even a reach like Mississippi opening up counts as a meaningful shift.
Here’s how everything will play out. Under Mississippi law, Republican Gov. Phil Bryant will appoint a new senator to take over for Cochran until a special election is held this November (concurrently with the regularly scheduled midterm elections). There is a catch, though: Special elections in Mississippi are nonpartisan; that is, party affiliations aren’t printed on the ballot, and — instead of party-specific primaries — all candidates will run in one free-for-all of a race. If no one gets a majority of the vote in the first round, the top two finishers will face off in a runoff election.1
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Make no mistake: Mississippi is still a very red state, and its inelasticity means any Democrat will have to work twice as hard to persuade the few gettable voters available while still turning out the state’s sizable Democratic base. But Cochran’s seat becomes only the ninth Republican-held Senate seat to be on the ballot in 2018, most of them on dark red turf. Democrats must pick up at least two of them (and not lose any of their own) to take a Senate majority. Mississippi is a long shot, but it’s one more shot than Democrats had on Sunday.
Is it likely that the Dems will flip this seat. Of course not. But they’ve already won a similar longshot Senate seat, and every seat in play helps.