Herman Cain is blaming liberals for the recent sexual harrassment allegations against him. I guess that's the thing to say if you're running for the GOP nomination, but obviously it would be dumb timing for any actual liberal to hand a bad story on Cain to the media right now. If you're a liberal, you'd sit tight and wait until Cain is the nominee to start dumping stuff on him. (I think Cain is extremely unlikely to be the nominee.)
This looks like the work of one of his GOP competitors. My top suspect would be the Perry campaign, just because it makes sense for them to do it -- they certainly don't want this Cain surge lasting any longer.