Mary Landrieu’s winning math = 50+1. Literally.

Over the next several months, we're going to see a lot of polling in key Senate races. But in one state in particular, you need to be reading the polls a little differently.

That would be Louisiana. The Pelican State has an election process all its own, featuring an open primary on Election Day and a runoff -- which appears likely -- in December. And we've already salivated over the idea -- however unlikely -- that such a runoff could determine control of the Senate.

In Louisiana, you will see two kinds of polls: one testing the entire field, including Sen. Mary Landrieu (D-La.) and three top Republicans running against her, and the other featuring matchups only between Landrieu and her likeliest runoff opponent, Rep. Bill Cassidy (R-La.).

Both are important, but the latter is probably a better gauge. And in all of them, it's probably best to focus on one number.

First, let's take a look at a brand new poll -- shared early with The Fix -- that was conducted for the Democratic group Senate Majority PAC by Greenberg Quinlan Rosner. It shows Landrieu with a 19-point lead on Cassidy in the open primary.

LandrieuPoll4

That 19-point deficit, though, isn't really a good gauge. Given the crowded GOP field against Landrieu, Republicans will continue to split up their votes through Election Day. Landrieu, then, will continue to lead this kind of poll -- unless she is in deep, deep trouble. The real question is how close she is to 50 percent.

That's because, one way or another, she's going to have to get to 50 percent -- either on Election Day or in the runoff. That might sound like a pretty obvious statement, but not all senators need to get to 50 percent plus one to win. In fact, in 2012, four senators won with half the vote or less.

This particular poll shows her with 48 percent of the vote, which would be good for her because she would need to get only two more points to avoid a runoff altogether.

(Other polling has shown her in the low 40s. And we will emphasize here that this poll was conducted for a group that supports her, so take it with a couple grains of salt.)

If you want to take the same poll and get a better idea for where the race stands, you just stack the Republicans on top of one another, assuming that the GOP standard-bearer in the runoff will likely get most of those votes.

LandrieuPoll1

In this case, that brings you to Landrieu 48, Republicans 44.

Here's another example, from the recent New York Times/Kasier Family Foundation poll. This one showed Landrieu at just 42 percent, but with a much bigger lead (Landrieu 42, Republicans 27) and many more undecided voters.

LandrieuPoll2

Which brings us to the second kind of poll you will see in Louisiana, and the most important one: the head-to-head runoff.

While a poll from another Democratic pollster in February showed Cassidy beating Landrieu 46 percent to 42 percent, the new poll for Senate Majority PAC shows them deadlocked at 49 percent.

LandrieuPoll3

When you compare the open primary poll to the runoff poll, Cassidy rises from 29 percent to 49 percent and actually takes more of the vote than the trio of Republicans did (44 percent) in the first poll. Landrieu, meanwhile, ticks up slightly from 48 percent to 49 percent.

That suggests that Cassidy -- or whoever the GOP nominee is -- will gain the vast, vast majority of the other non-Landrieu voters in a runoff scenario.

Which means the name of the game for Landrieu is getting as close to 50 percent on Election Day as she can.

Aaron Blake covers national politics and writes regularly for The Fix.
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