Let's get one thing out of the way at the outset: No one really knows what's going to happen in today's run-off election between Sen. Thad Cochran (R-Miss.) and Chris McDaniel, his conservative challenger. But that doesn't mean we don't know where to look.
The Upshot's Nate Cohn explains the key factors behind that uncertainty. In short: Everything that campaigns do to prepare for Election Day hinges upon having a clear understanding of what the electorate will look like. Usually this isn't too hard to figure out, since voters tend to vote in predictable patterns. Past elections provide good insight since turnout percentages often remain similar over time. Polling offers another layer of information, allowing campaigns to evaluate who's most likely to actually head to the polls.
This Mississippi run-off is very different. Cohn points out that 37 of the 40 Senate run-off elections since 1980 have seen decreases in turnout from the initial primary, reflecting the difficulty in getting voters to care about a primary election two times in a row. But interest in Mississippi has likely grown since the June 3 primary, at least on a national level. It's become a proving ground for some Tea Party groups, as the Post's Ben Terris reported on Monday. It's an unprecedented race in Mississippi at an unusual time in American politics. On top of that, add the deliberate effort by Cochran's camp to turn out more black voters, mixing up the expected voter pool. That makes predicting turnout tough.
But it's also a race that didn't offer many clues in the first place. Thirty-nine of the 82 counties were settled by a margin of 200 votes or fewer in the primary, in a race that ended up handing McDaniel the first-place position by fewer than 1,400 votes. Had those counties each moved 40 votes in the other direction, the results of the race would have been flipped.
OK. With all of that said, here's what you can watch for.
This is a graph of the vote margin in each of the state's counties in the primary. That long flat stretch in the middle shows those 39 counties mentioned above. But pay attention to the spikes at either end.
At the far left is Jones County. Calling it a stronghold for McDaniel doesn't really do it justice. He won the county -- which he is from -- by a 71-percentage-point margin. If McDaniel had won the county by a margin of 75-25 percent (instead of 85-14), Cochran would have come in first place.
It's not clear that Cochran will pick up many new votes in Jones County. But lower turnout could be helpful to Cochran. The county had the seventh-highest vote total in the primary, despite being the tenth-largest county in the state. If turnout declines, that will certainly help the incumbent.
Now look at the other end of that chart above. At the far right is Hinds County. It's where Cochran did best in terms of vote margin. Hinds County also had fourth-highest vote total -- despite being the state's most populous county. It also has the state's highest percentage of population that is black. If Cochran can increase turnout in Hinds (which is also more Democratic than much of the state), it could be significant.
But again: This is how Cochran or McDaniel could have won on June 3. What will likely happen on June 24, today, is an entirely different question, one for which we don't have a very good answer. Maybe turnout will plunge. Maybe it will be massive. We'll see. If it's anything like the primary, though, Hinds and Jones are the counties to watch.