Of late, Bernie Sanders has taken fire for dismissing Hillary Clinton's delegate lead as a function of her wins in "the Deep South," which he reminds audiences is "a pretty conservative part of this country."

This is neither particularly true nor particularly relevant. Clinton has won in Illinois and Florida, among many other states outside the Deep South, and even if she hadn't, delegates from the Deep South count as much as delegates from anywhere else.

But in a week or two, Sanders might have a new, and more potent, argument. As Ron Brownstein notes, Sanders dominates with independents who vote in Democratic primaries, but of the next six primaries — New York, Connecticut, Delaware, Maryland, Pennsylvania, and Rhode Island — only Rhode Island allows independents to vote in the Democratic race.

Making Sanders's life even harder, some of these states don't allow independents to re-register as Democrats on or near the date of the election, either. In New York, for instance, independents who wanted to vote for Sanders had to make that decision and change their affiliation back in October 2015. So if you're an independent in Buffalo, New York, who was inspired by Sanders's performance in last week's debate, well, too bad. That hurts Sanders's chances, and he knows it.