Correct me if I am wrong, but didn't the Republicans start as a third party, or have I flipped my Whig?
I understand the point you are making... the Republican party being formed to endorse (in general) anti-slavery and free land. The Whigs went away, but the Dems stayed. The current two party system has been entrenched for over a century and people's perceptions are pretty much stuck in viewing our system in this manner. Some may refer to them as "sheep", but so what? Tough turtles, because that's the way things are. A more realistic way to promote change is to alter the Republican party from within rather than try to take the more "likely to fail" route IMHO.