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Bernie Sanders desperately needs to shrink Hillary Clinton’s delegate lead in the Democratic presidential race, but that is going to be hard to do in Wisconsin even if he wins by 5 or 10 points today.

There are two reasons why. Democrats allocate delegates here proportionally, not winner-take-all. And they allocate the majority of delegates by congressional district. For an illustration of how this works, take a look at the district-by-district delegate breakdown below, from Wisconsin’s 2008 Democratic primary (the statewide delegate allocation is the last row). Clinton lost by 17 points to Barack Obama, but still won 32 of 74 pledged delegates. In other words, Obama only “netted” 10 pledged delegates, despite a fairly massive popular vote victory, and despite winning all 8 CDs:

You can see how proportional allocation within congressional districts can result in an even split of a district’s delegates even when one candidate has a clear victory. Take a look at the Third District, which had six delegates. Obama won it by 14 points. But he only got three of the six delegates because that was closer to his proportion of the district vote (56%) than receiving four of the six delegates would have been (a 67% percent share). Something similar happened in the Eighth, Seventh, and First, which Obama won by anywhere from 5 to 13 points but only got half the delegates in each. It’s just the way the math works when you’re using proportions of very small even numbers, like six.

Consider one possible scenario tonight consistent with most of the polling – a statewide Sanders victory of 5 to 10 points. The First, Sixth, Seventh and Eighth districts each have six Democratic delegates this time around (the party apportions delegates based on how Democratic a district is, and the delegate counts are a little different in 2016 than they were in the 2008 example above). The delegates from these four districts are very likely to be evenly divided, for the reasons explained above.

The Fifth District (the very Republican areas outside Milwaukee) has five delegates. Let’s give Sanders three out of five under this scenario, for a net gain of one.

The Third District (in western Wisconsin) has seven delegates. It would take a huge Sanders vote margin to capture five of the seven; let’s give him a four to three edge, for a net of one.

The Fourth, in Milwaukee, has 10 delegates. Most of the state’s African-American population is in this district, and that’s Clinton’s best voting group. There are also a lot of white and some Latino voters in this district. Let’s say these delegates are divided 5-5 (though a big Clinton margin here could give her 6 and Sanders 4).

That leaves the Second, anchored by Madison, which has 11 delegates. If Sanders replicates Obama’s big margins here in 2008 (65% of the vote), he would win seven out of 11 delegates, for a “net” of three.

Add it all up, and that’s a net gain of just five delegates out of the 57 in Wisconsin allocated by congressional district.

The state also allocates 29 delegates based on the statewide result. It would take about a 14-point win statewide to win 17 of the 29, for a net gain of five. It would take about an 8-point victory to win 16 of 29, for a net gain of three.

To sum up, even a 10-point victory by Sanders would probably “net” him no more than three statewide delegates and five district-level delegates, for a net gain of eight (47 out of 86 pledged delegates). It could easily be less than that, too. A very impressive 14-point statewide victory might net him 10 (48 out of 86).

And that doesn’t include the state’s 10 unbound super-delegates, some of which are uncommitted right now, and some of which are pro-Clinton.

Bottom line: a Sanders victory today could give him a momentum boost going forward, but it won’t give him a big delegate boost.

Follow Craig Gilbert on Twitter @WisVoter