Party registration isn't required (I'm unaffiliated, for example) but enough people do it that you can make a reasonable prediction of how a precinct is going to vote before they actually vote. This is the input data for gerrymandering: you don't need to know every single voter, as soon as you get a statistical sample you know how the area is likely to vote, and then you can construct districts out of precincts such that there's a safe margin of victory for each one.