Perhaps, I'm just bitter from the NJ marriage fight, but over and over again, the message seems to me that the fight has to go to the public, not to their representatives.
If one can show they can move public opinion or raise the perceived importance of an issue, politicians will respond. Hoping for "leadership" is nice and sometimes it happens, but ..
Anyway, how much does a national billboard campaign cost?
Look what these people have done. Expanded in a concerted way (focused on states with Senators who may be 'leaning' or swing states), it could have great merit. What's more, once you have the 'adopt a billboard' mechanics in place, you can use it over and over again, for messaging re-enforcement.
I did some guesstimated - very guesstimated - calculations. It might take 375,000 people willing to give $200 dollars, assuming some matching funds could be found.