Suppose that a beloved artist/band announces that they will release a new album the day after Election Day, but only if their chosen candidate wins. Otherwise, the album will be destroyed. Let's assume that the promise is made with enough time to register to vote. Who would have the most influence? Who has the most voting-age fans who give enough of a shit to register and make it to the polls, but not enough of a shit to respect the democratic process? Beyoncé?
Beyoncé. I can't even think of anyone who would come close. Every white girl in her twenties would go fleeing to the polls, which would give the Democratic candidate a significant boost in terms of voter turnout.
In fact, if I were the Democratic candidate, I would do exactly this. I would set up a Super PAC and funnel $10 million to Beyoncé (do Super PACs work like that? Probably not), and then I would ask her to make a video to be released on midnight of Election Day, but it only gets released if I win (provided Beyoncé agrees with my liberal pledge to give every American a free puppy and institute a tax on misspelled web comments). Then I would sweep the coastal states, carry the Midwest, defeat the GOP, and Beyoncé would unleash a five-minute video of herself dancing with a chair and singing WAY too fast, and people would lose their goddamn minds. That's the kind of modern electioneering you have to look forward to in 2016. [source]
The Smiths. Sweep the Latino vote.