Sam Smith of the Progressive Review lays the case here for why the GOP lock is not nearly as bad as it seems. He blames Democratic misfortune largely on what he calls the "Vichy Democrats" of D.C. who try to position themselves as GOP-lite (responsible Republicans) instead of populists. When both parties sound about the same, swing voters don't see a difference -- because the "Vichy Democrats" work hard to erase any significant difference between the parties. That is a poor strategy.
In the base GOP states, income is below-average. That's an opportunity for populists to earn votes. Anyway, check out the article.