According to The Huffington Post, the Hillary Clinton campaign released this video demonstrating the support of Jack Nicholson:

UPDATE: Over at his The Chutry Experiment, Chuck Tryon explores the semiotics of this advertisement, noting that many of Nicholson’s characters are rebellious anti-establishment types, and muses on the collision between these associations and the Clinton campaign. Meanwhile, The Caucus blog at the New York Times discusses this video as well and, as a poster in the comments thread below concludes, doubts that it came from the campaign itself.