There's a lot of this floating out there. Questions about Sanders not raising money for Democrats down-ticket. Questions about his voting history. His past assertions that the blue party wasn't really that different from the red party. His more recent assertion that he only ran as a Democrat to get media coverage.
After that last comment, we looked at the extent of independent support in the Democratic primaries and caucuses -- and it's not insignificant. In a number of states -- Alabama, Arkansas, Massachusetts, New Hampshire and Vermont -- more than 45 percent of his support came from people who identified themselves as independents to exit pollsters. In most states, at least a third of his support has been from independents.
Bernie Sanders won Wisconsin by about 13 points -- but he and Clinton tied among Democrats. Sanders ran up huge margins among independents -- in the same way that Clinton has won states thanks to her disproportionately high support from black voters.
You can see the effect of Sanders's strength with independents when you look at the votes that came in. Compare the light blue slices (Clinton's support from independents) with the light yellow (Sanders's). The latter is a much larger slice of the pie than the former.
We can illustrate this another way. Democratic delegates are given out mostly proportionally (though, since this often trickles down to the congressional district level, it can get a little wonky). If we take the states for which we have exit poll data and excise the independent vote completely, we can get a sense for what the delegate count in the states would look like if only Democrats were weighing in on the Democratic contest.
In Wisconsin for example, Clinton and Sanders would have split the delegates instead of Sanders netting a plus-11 margin. In other words, Clinton's wide delegate lead wouldn't have fallen at all, if only Democrats voted.
We can go further. If we tally up the changes in each state in the graph above, Clinton's delegate margin from the states shown jumps from plus-337 to plus-604. In other words, if you take out independent votes, Clinton's pledged-delegate total jumps from 1,279 to somewhere over 1,400. Her overall lead (including states for which we don't have exits) goes from 252 to more than 500. If the former was insurmountable, which it essentially is, the latter is Everest.
But that's not how it works, to Hillary Clinton's consternation. The strong support of independents coming out to vote for a long-time independent is making the Democratic front-runner have to keep scrambling to win Democratic primaries. Clinton doesn't even know if the guy is a Democrat! People coming to vote in the Democratic primaries don't seem to care.