How do presidential primaries serve to test the electability of a candidate?

It measure's public perception toward the candidate

Explanation:

During the presidential primaries, different candidates within a same political party will be presented to the public. The public can cast their vote to select which candidate will represent the party in the real presidential election.

The party can use the result of the primaries to measure how popular is the candidate within their own voter base. If the candidate manage to obtain the vast majority of vote during the primaries, the party can be sure that the candidate will be able to unite the voter base and forecast how much loyal voters will come out during the election.

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