Well, it largely depends on whether or not gay marriage has lost its shock value. It also depends on whether or not the Dems are actually prepared for this and handle it well. Plus the Foley scandal provides an interesting backdrop to this story, as it has certainly shed some light on the fact that there are far more gay Republicans on the Hill than previously thought. (Well, unless you live in Washington, then it was hardly a surprise.) The GOP can proclaim their ignorance of all things homo but they're running a bit low on plausible denial.
At this point how this goes is anyone's guess. It could certainly reinvigorate the right-wing base but it probably won't have much impact on the 76% of independents who are strongly favoring Dems these days. It certainly won't cost the Dems any blue voters. I think it's also possible that a "let's throw the bums out" wave could very well overpower any dire Chicken Little warnings about teh gays destroying America by (gasp!) getting married.
Mostly this feels like the final test of how much most people care about gay marriage and the average American's ability to prioritize important issues (life, death, health) from the not-so-imporant issues (whether or not Heather and Jen down the street can get legally married). Let's hope America passes with flying colors.