In two-dimensional two-party electoral competition under plurality rule, there are typically no equilibria, even when one of the dimensions refers to valence. The good news is that the introduction of either closed or open primaries acts as a stabilizing force since equilibria exist quite generally, serves as an arena for policy debates since all candidates propose differentiated platforms, and guarantees that each party's nominee is of higher quality than its primary opponent. Moreover, primaries tend to benefit the party whose median voter is closer to the overall median. The bad news is that the winner of the general election need not be the candidate with the highest overall quality since too competitive primaries can prove harmful. Given the differences between open and closed primaries, we show that the choice of primary type is particularly important and may determine the winner of the general election.