Beginning today in Michigan, through February 5, there will be 29 state primaries and caucuses. However, in 17 of them, in order to vote for one of the national candidates, one must be a registered member of either the Republican or Democratic parties; these are the so-called "closed primaries."
This is one of the more effective tools our established political parties have employed to entrench themselves at the controls of our government. It's undemocratic and a major obstacle to any progressive candidate, from either party, who wishes to appeal to independents and moderate members of the opposition party.
Closed contests are held in some of our biggest states including California, New York, Massachusetts, Florida and New Jersey.