In the US, people from either the Democratic or Republican Party who want to run for president must go through primaries, where the respective party votes for their presidential nominee.
The same process goes for elections to the House or Senate –- party supporters first vote for their preferred candidate in the primaries, then the chosen Republican and Democratic candidates face off in general elections. Americans also generally identify themselves as either Democrat, Republican, independent, or from other minor parties — they don’t have to be card-carrying members.
This is unlike Malaysia, where aspiring candidates need a “watikah” from their party president before they run for state assemblyman or Member of Parliament.