Early parties were electoral coalitions of elites, mostly in the U.S. Congress. They were mostly designed to help win House elections and the presidency, but they quickly expanded activities to the state level.
Third parties bring important issues to the attention of the major parties. They also often serve as spoilers in the elections they enter.
Parties can’t influence and enact policy without winning. They must organize at each level at which elections take place in order to contest elections and develop candidates.
The sorting thesis says that voters change party allegiances in response to shifts in party position. It suggests that polarization is a function of voters’ paying more attention to national politics and voting more consistently.
They have pulled their respective parties further to the ideological poles and have changed the issues parties consider. They may also have made compromise more difficult.