In our winner-take-all system, the natural constituency for a third party would have to be willing to withdraw their support from the less unfavorable major party candidates, and give elections over to the other side's nutjobs. That's a very high cost just to communicate a trend to politicians, which could have been done by straw poll without the resulting damage to civil rights, the economy, church-centric values, or whatever you're most concerned about. The cure for tactical voting is some other polling system like IRV or approval, where I don't lose all possible effect on outcome just because my favorite doesn't win.