Electoral coordination occurs at two main levels: (a) within individual electoral districts, where competitors coordinate entry and citizens coordinate votes; and (b) across districts, as competitors from different districts ally to form regional or national parties. We know a fair amount about district-level electoral coordination for single-tier electoral systems. In particular, when political actors are primarily concerned with the current election and have good information about the relative chances of potential competitors, two different M+ 1 rules apply in an M-seat district. First, the number of competitors entering a given race tends to be no more than M+ 1; second, when more than M + 1 competitors do enter a race, votes tend to concentrate on at most M + 1 of them. We know much less about cross-district coordination, in which potentially separate local party systems merge to form a national party system. This essay focuses on the latter, relatively neglected topic.
