This piece in Politico urges Bernie Sanders to make a campaign issue out of the fact that in “half the states”, independent voters can’t participate in the process by which the Democratic Party chooses a presidential nominee. See the third paragraph from the bottom, in which the author, Jackie Salit, says “New Hampshire permits independents to vote. Half the states don’t.”

Actually, only twentyone states prevent independents from participating in the Democratic presidential caucuses and primaries. See this page from Bernie Sanders’ webpage, explaining the procedures in each state.