In Yours Truly vs. the Governor of Florida Davis Byrne explains that he's bringing a lawsuit against the Governor of Florida.
Well, using a recording of a song, or even just using that song and not the original recording, in an advertisement without permission is illegal, unless the composition has gone into the public domain. It’s not just illegal because one is supposed to pay for such use and not paying is, well, theft — it’s also illegal because one has to ask permission, and that permission can be turned down.
Besides being theft, use of the song and my voice in a campaign ad implies that I, as writer and singer of the song, might have granted Crist permission to use it, and that I therefore endorse him and/or the Republican Party, of which he was a member until very, very recently. The general public might also think I simply license the use of my songs to anyone who will pay the going rate, but that’s not true either, as I have never licensed a song for use in an ad. I do license songs to commercial films and TV shows (if they pay the going rate), and to dance companies and student filmmakers mostly for free. But not to ads.
I’m a bit of a throwback that way, as I still believe songs occasionally mean something to people — they obviously mean something personal to the writer, and often to the listener as well. A personal and social meaning is diluted when that same song is used to sell a product (or a politician). If Crist and his campaign folks had asked to use the song, I would have said no — even if they had offered a lot of money, such as I have been offered in the past for ad use (though I’ve always turned these offers down).
Repeat after me: get all rights to music before you air anything. It's advertising 101.