This article on Wired from 1996 says that they are prohibited from taking a political stance with the non-profit status (never identified as 501c3):
Rock the Vote was established as a nonprofit, meaning that it can't take partisan political stances without running the risk of having its tax-exempt status yanked. But now that status appears to rest on a razor's edge. In this week's issue, the conservative Weekly Standard presents a harsh indictment of what it sees as RTV's leftist political bias.
Here is the current thread on the bias of Rock the Vote:
Rock the Vote Betrays 'Non-Partisan' Label
And here is the illegal advocacy banner ad that I saw on their site. They are prohibited from telling visitors to vote Yes/No on any ballot initiative or for/against any candidate:
Someone needs to report this to the IRS if it can be shown just what their non-profit and tax exempt status is (from what I can find online, they were still tax exempt in 2000).