primary election; preliminary election
Refers to an election held to choose a party's candidate for a main election, common in political contexts.
In the U.S. presidential election, each party's primary election is held first.
As a result of the primary election, he was chosen as the party's candidate.
予備選挙 is the clear, standard term for a political primary election, while 予選 in this sense feels abbreviated or informal.
Compound of 予備 (preliminary, reserve) and 選挙 (election).