Third party = political oblivion.
The GOP is a third party. They replaced the Whigs.
At this point, I'd settle for having a second party, know what I mean?
After which (not immediately after, mind you, but later on), the Republicans and Democrats came up with ways to tweak the system so that no other third/minor party could upset the two-party apple cart.
Or to put it another way -- if conservatives don't have the numbers and resources to take over the GOP, then they don't have the numbers and resources to take on the GOP and DNC combined.