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History Really Was on Cochran’s Side in the Mississippi Runoff

Political junkies analyzing Sen. Thad Cochran’s triumph in Mississippi’s GOP primary runoff can learn many lessons about the state of politics today. Those lessons include how the Mississippi result applies to the larger establishment vs. tea party argument, the importance of get-out-the-vote operations, and the relationship between black voters and the Republican Party.

Let me throw in one more lesson for those in the business of election prediction: When you’re doing political analysis, it’s important to focus on the right data, choose the proper historical precedent and be wary of small sample sizes.

When it was clear that the Senate primary was headed toward a runoff, some analysts tried to understand how such a race would shake out by looking at previous runoff results. Because there’s a history of poor polling in Mississippi, this technique seemed to make sense. The problem was, there were so few examples nationwide of Republican runoffs for governor or Senate, and even fewer of those involving incumbent senators. In the past 25 years, Democrat Blanche Lincoln of Arkansas had been the only incumbent senator to face a runoff. Additionally, Fob James of Alabama had been the only incumbent Republican governor involved in a runoff.

This led number-crunchers to examine runoff results in races for lower office, such as the House of Representatives, or those not involving an incumbent. But while that search may have cast a wider net, the findings also led analysts to dismiss Cochran’s chances too easily, and, when he won, to assert that Cochran defied history.

The idea that history didn’t favor Cochran generally sprang from one of four hypotheses, which I’ll outline below. As we’ll see, those hypotheses fail to hold up when we take a more narrow look (like the one I conducted just after the initial primary) at the proper precedents: Senate incumbents and others involved in Republican gubernatorial and senatorial runoffs, particularly Lincoln and James.

Idea 1: Incumbents lose ground between a first round and a runoff.

Although many incumbents for lower office who face runoffs have lost vote share between the first round and the runoff, neither Lincoln nor James did. They gained ground. Either of their percentage-point gains against their opponents would have been enough for Cochran to defeat the more conservative Chris McDaniel.

Idea 2: The more ideologically pure candidate gains ground between a primary and a runoff.

This may happen in races for lower office, but Lincoln and James were more moderate than their opponents, according to Stanford professor Adam Bonica’s ideological scores. In fact, in all 13 Republican gubernatorial and senatorial runoffs of the past 25 years — including non-incumbents, the more conservative candidate picked up vote share only one more time than the more moderate candidate. Of the Republican incumbents, Cochran makes it three for three on improved vote share.

Idea 3: Cochran was cooked because he got fewer votes than his opponent.

Simply put, there was no real history here. There hadn’t been any Senate incumbent of either party or even a Republican gubernatorial incumbent who’d made it to a runoff while getting the second-most votes in the first round in the past 25 years. In eight of the 13 Republican primaries that did go to a runoff, the candidate for governor or senator trailing in the first round gained vote share. Cochran joins this group.

Idea 4: Cochran would have a difficult time bringing more voters to the polls.

Just before the Mississippi runoff, I wrote that Cochran needed to expand the pool of voters to advance to the general election. Since 1980, turnout in Senate runoffs increased only three out of 40 times. But as Nate Cohn at The New York Times pointed out, most of those runoffs were not tantamount to a general election victory, as Cochran’s was in Mississippi. Only one of these elections involved an incumbent: Lincoln in 2010.

If we looked at only Republican incumbents running for major statewide office in the past 25 years, we’d have one case: James in 1998. Turnout in his Alabama race was up 28 percent. This is close to the 17 percent increase we saw in Mississippi this month.

All of this is easy to see in retrospect. But even before Tuesday night, the facts were there to question the idea that history was against Cochran. Republican runoffs for major statewide office are rare, and even rarer for incumbents like Cochran.

Harry Enten was a senior political writer and analyst for FiveThirtyEight.