It’s primary day again! Alabama, California, Iowa, Mississippi, Montana, New Jersey, New Mexico and South Dakota are all holding elections Tuesday. But of those, the Republican primary for U.S. Senate in Mississippi is the most interesting.

Here’s what you need to know:

The candidates

The tea party vs. establishment rubric is overused in political journalism, but this is one case where it mostly applies. Incumbent Sen. Thad Cochran is vying for a seventh term (he was the first Republican senator Mississippi elected in the 20th century). And Cochran is, in many ways, an old-school GOP senator. Backed by the Republican establishment in Mississippi and Washington, he has a reputation for bringing the “pork” home and a relatively moderate voting record compared to his Republican colleagues in the Senate.

Cochran is being challenged by state Sen. Chris McDaniel. McDaniel is backed by most tea party groups, the Club for Growth and conservative luminaries Sarah Palin and Rick Santorum. A win by McDaniel would be the first clear tea party victory in a competitive Senate race this year.

The Pollster aggregate has Cochran at 43 percent. McDaniel is at 42 percent.

Although not frequently mentioned, a third-party candidate, Thomas Carey, is also running in the primary. He has averaged 3 percent in the polls that have included him.

The rules of the game

A candidate must receive more than 50 percent of the vote to avoid a runoff, and Carey may take enough of the vote to keep both Cochran and McDaniel below that threshold. If there is a runoff, it will be held June 24.

What this means for control of the Senate

The winner of the primary will almost certainly face Democrat Travis Childers. Childers, the former congressman for the deeply red 1st Congressional District, is an underdog against both McDaniel and Childers. That’s because, as I outlined previously, Mississippi is inelastic: 80 percent of white voters consistently go Republican, 90 percent of black voters go Democratic and there are many more white voters.

Still, Republicans hoping to take back the Senate probably prefer Cochran as the stronger general election candidate. He has polled at least 5 points better than McDaniel in the few general election surveys and has never received less than 60 percent of the vote since winning his first term. McDaniel is also new to the political scene and mostly untested (a recipe that doesn’t always work well). Some McDaniel supporters have been arrested for breaking into a nursing home and taking pictures of Cochran’s wife, who suffers from dementia.

Past polling accuracy

There have been very few high-profile, competitive Republican primaries in Mississippi over the past decade. The Republican presidential primary in 2012, however, suggests we shouldn’t put too much stock in the polling. In that primary, Mitt Romney led Newt Gingrich by 5.4 percentage points and Rick Santorum by 8 points in the polling average. Santorum ended up beating Gingrich by 1.6 points and Romney by 2.1 points.

Current problems with polling

Indeed, pollsters seem to have different ideas of who is going to vote. The past four polls released publicly had wildly different estimates of the 65-and-older vote, from 33 percent in a Polling Company survey to 58 percent in a Harper Polling survey. That’s a big deal because Cochran is expected to do better with older voters. The Polling Company poll had McDaniel up 4 percentage points; Harper Polling had Cochran up 5. In the 2012 primary, voters 65 and older made up 33 percent of the electorate, according to exit polls.

Given that this is a non-presidential year and exit polls typically estimate a higher percentage of young voters than other surveys, older voters will probably make up 35 to 40 percent of voters. Polling in the South has also had a tendency to underestimate the more conservative candidate. In other words, Cochran’s 1-point lead in the polling average isn’t worth much. I might actually say McDaniel is the favorite, but who knows? Polling in the South is a mess.

Bases of support

Past results and polling suggest that Cochran’s main support is likely to be in his home county, Lafayette, in the north; the wealthier and more populated Hinds, Rankin and Madison counties around the city of Jackson; and Jackson and Harrison counties along the Gulf Coast, which rely heavily on federal dollars.

McDaniel will count more on southeast counties north of the coast, such as Jones, where he lives, and Forrest. As Slate’s Dave Weigel pointed out to me via email, a lot of the overall vote should be coming out of the southeast part of the state because of a competitive Republican congressional primary. History also suggests that the northeast corner of the state, anchored by heavily populated Lee County, should be prime territory for McDaniel.

Key county

Although we have limited polling, DeSoto County, which borders Memphis, Tennessee, could be telling. Non-tea-party-aligned Republicans tend to do better better in more highly populated areas, but a lot of the voters in DeSoto are originally from out of state and have no long-term ties to Cochran. Romney and Santorum came within a few percentage points in DeSoto of how they did statewide. If Cochran is winning DeSoto, he is probably winning the primary.