At present we have two parties that are both owned, for the most part, by the same people, and kept in power by gerrymandering and the systemic weakness of first-past-the-post elections.
Further, the people in control of the major parties themselves cheat when someone not of their faction tries to go the primary/caucus root. They change rules in midstream, miscount, break meeting rules, physically attack supporters of opponents, pass out bogus delegate slates, and a host of other dirty tricks.
For a list of the things the Republican have done to just one challenger in the last two cycles, check out the archives of any of the several sites where Ron Paul supporters congregate. (For example, The Daily Paul.)
The Democrats do this as well. (The riots in Chicago in 1968 were largely a public reaction to the party machine repelling a primary effort by Gene McCarthy, popular with the antiwar movement, in favor of Hubert Humphrey. The Paul/Romney nomination battle was eeriely similar.)