Baseball players, boxers, politicians: each is a contender in that they compete or campaign to win something.
Contender has a slightly different meaning than competitor, often implying a more vigorous, even scrappier, sense of taking on every unexpected challenge. The most famous use of contender is Marlon Brando's quote in the film On The Waterfront. Brando plays a once promising boxer whose career has been ruined by a fixed fight. "I coulda been a contender!" Brando cried, implying he could have been the boxer whom everyone wanted to defeat.