The expectation follows directly from long-standing and widely influential theories of political competition that are collectively referred to in their simplest form as the "median voter theorem" (Hotelling, 1929;Black, 1948;Downs, 1957).
it is to be expected that when a set of voters that are more ideologically concentrated are replaced by a set who are more widely dispersed across the ideological spectrum, as occurs in the transition between the United States primary and general elections, that candidates will present themselves as more moderate in an effort to capture enough votes to win.