I've always been under the impression that the two party system exists in an equilibrium and it naturally goes to about a 50/50 split. Both parties are fighting for support so if one party gets more than 50% of the vote there is a huge incentive to shift your positions to get back to equilibrium. Imagine if there were policies that would get Republicans reliably 70% of the vote, in this situation Democrats would want to adopt very similar policies and restore the balance. I think you'll see this happen with gay marriage in the next decade or so where both parties are for it.