We now have exit poll data from 27 states, a group that tends to favor Clinton because it excludes a number of smaller and caucus states. In those 27 states, people who identify as independents — which is what this is, not actual party registration — have backed Sanders 24 times. People who say they're Democrats preferred Clinton 24 times.
West Virginia's results fit into an elite subcategory: states that Sanders only won thanks to non-Democrats.
This is admittedly an estimation, based on fiddling with exit poll numbers that are themselves weighted approximations of the composition of the electorate. But taking that data and splitting up how much of the vote in each state came from Democrats and how Clinton and Sanders fared with each, we can put together a few different categories of states.
In the three states where Clinton didn't win the majority of Democrats, Sanders won each time. That was two blow-outs in Vermont and New Hampshire and his win in Wisconsin, where the two tied among Democrats. Wisconsin, then, falls into the category of "won thanks to non-Democrats." After all, if you tie with Democrats but win anyway, it must be the non-Democrats that got you over the finish line.
There are three other states in that group with West Virginia and Wisconsin, according to our analysis: Indiana, Michigan and Oklahoma.
In four other states, independents helped Sanders stay close to Clinton. Clinton barely won Iowa and Missouri, but if the vote had been closed to non-Democrats, that likely wouldn't have been the case. (Since this data includes only self-identification, it's hard to know how many people who say they're independents actually are.)
Most of these totals don't add up to 100 percent, thanks both to rounding and to the fact that we're excluding people who identified themselves as Republicans. Yes, that's something that happens. Only a handful of respondents usually say they're Republicans voting in the Democratic primary, 3 percent on average. But in West Virginia, according to preliminary exit poll data reported by CNN, that figure was three times as high. (In Oklahoma, it was 7 percent.) Those self-identified Republicans are probably also why so many Democratic voters plan to vote for Trump.
If the Democratic primary had included only people who refer to themselves as Democrats, Sanders would have lost the five states in the lower right of that graph. The average margin of support for Clinton from Democrats in states for which we have exit poll data is 63 percent; had she won that percentage of pledged delegates in the party's proportional-allocation system, she'd have already clinched the nomination.
But that is not how the process works. So on Tuesday, thanks to people who don't consider themselves Democrats, Bernie Sanders racked up another win and another argument for staying in the race. His impressive run is all the more impressive when you consider that he, too, was an independent that voted with the Democrats only a year ago. A guy who's been in the party for about 12 months has won more Democratic delegates than anyone not named Clinton, Gore, Obama or Kerry in the past 25 years. And he can thank other Democrat-voting independents for that distinction.