Potentially millions of Illegals voted. (potentially) http://www.investors.com/politics/editorials/trump-is-right-...
Here's one illegal that voted FIVE times. http://www.nbcdfw.com/news/local/Trial-Begins-in-Alleged-Ill...
AND the DHS supposedly under Obama administration tried to hack electronic voting polls for six states. http://www.zerohedge.com/news/2016-12-08/georgia-accuses-hom...
EDIT: I only said supposedly on the third because SIX is the supposed number, but the Georgia one DID happen.
The winner of the US presidential election must be elected by majority vote.
Of course, as long as we're hypothesizing the election to be by popular vote, we could make it by plutority too...
It's up to each state to choose their electors (plutority of popular vote, as you said being the most common, though not the only method).