[Edit] I meant targeting the reps that are voting for SOPA, not against it.
Would it really damage advertising revenues that much? If the fear is that Google would be hit by large lawsuits from some twelve year old posting a music video on YouTube, or that Facebook would be liable for a link to it, I expect that media conglomerates would be happy to give de facto immunity, so long as Google or whoever is willing to put Viacom's commissars in charge of which content is allowed and which isn't.
Obviously it's good that Google is on the side of angels on this, but if it's actually in their best interest is a relevant angle for analysis. It also would tell us how hard they're actually willing to fight to block SOPA.
There's a big problem with patent trolls right now making totally frivolous claims based on patents that they bought up, right? And yet many companies are paying oodles of dollars to pay them off and to try to avoid future trouble from essentially ridiculous patents. I would expect exactly the same thing to arise under SOPA, but with ridiculous copyright claims instead of ridiculous patents. Entities with no actual concern about their IP will start knocking at the door looking for shakedown money when it allegedly gets posted on Youtube.
SOPA would dramatically (and negatively) reshape the environment in which new services and business are developed (by startups, individuals, teams within larger companies, etc.). In the long run, that will matter more for the bottom line than anything else.