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.
Rocko, The use of the plural in elecciones works the same way in French. You have explained it very well, no need for me to elaborate.