Most states hold partisan primaries: registered Democrats vote for Democratic nominees, Republicans for Republican nominees. Some states "open" their primaries to any voter; others use "top-two" (CA, WA), "top-four" (AK), or full nonpartisan (NE legislative) designs in which the top finishers across all parties advance to the general.
Reform advocates argue that closed primaries empower the most ideologically committed minority of each party — rewarding extremism — while open or nonpartisan primaries push candidates toward the median voter.
Critics counter that parties have associational rights to choose their own nominees, and that nonpartisan systems may shut out small-party candidates from general elections entirely.