https://books.google.com/ngrams/graph "ganó las elecciones" is only about 5 times more common in Spanish books than "ganó la elección" which means both are likely used. But undoubtedly since it is more likely we would say a person won a single election in English and the inverse is happening in Spanish books it must mean Spanish does in fact use the plural often meaning a single election in the English sense of the word. I think some one commented before that votes in different regions/cities/states/precincts/etc can be thought of as separate elections (he won Florida, and Texas but not New York) and the combined results can be thought of as plural.
Just as a different way to look at it, elegir means to choose or select; after all the voters have made their choices or selections or elections the results are compiled, and it takes a bunch of these individual elections to get the job. But the easier way to look at it is that elections are like vacations--singular in English but plural in Spanish.