This is a technological solution for something which already has a perfectly-functional social solution: that's what backroom chats and dinners with lobbyists are *for* -- sussing out politicians for their private views. And don't forget the politicians' various aides, who can be counted upon to say things such as, "Well, I don't know if the Senator will commit to this, but I think you are doing wonderful work," or "he would probably agree". Politicians are also liable to tell the frank truth to inquiring constituents, when certain that they can not be publically held accountable for their private assurances. When enough private commitments have been made, a public flag is flown, and each politician assenting to the public proposition does so with a fairly clear knowledge of which other actors have made private declarations in support.

Further, there's always the mechanism of holding "public meetings on issue X" where Politician does not publicly commit to a stand, but professes a desire to "be informed by my constituents" and/or "researchers" etc. And don't forget "Congressional inquiries into issue X" where all the people with strong feelings about X can get together and feel out the positions of each other.

In short, social interaction provides many means of assessing the group's feelings without necessarily publicly disclosing your own, or to privately state a preference. The only need for a technological mechanism would be to counter geographic separation or some other breakdown in normal social interaction. The real enabling technology is simply "repudiatable speech", aka "speaking off the record".