The Brennan Center has this press release.  I have not yet had a chance to look at the methodology, but I am skeptical of the point made in a second, emailed press release, that Super PAC spending will actually cause turnout to decline.  (That part of the press release states: “The poll, conducted on behalf of the Brennan Center for Justice at NYU School of Law, also finds that one in four Americans are less likely to vote this year due to fears that candidates cater to the interests of super PAC donors over the public interest.”)

I think that’s no more likely than the hypothesis that turnout will go down in states without tough new photograpic voter identification laws.