from The Republican and Democratic brands endure, even if their core beliefs don’t- from Jeff Jacoby at Boston Globe

Yet in the end, political parties — or at any rate, big national parties — are about winning elections. And as society changes, so does a party’s ideological orientation. The transformation can be gradual, or it can be dramatic; sometimes party members embrace a new doctrine by degrees, sometimes intraparty factions clash in a battle royale. Either way, parties evolve. If you’re a partisan loyalist, a Republican or Democrat before anything else, you’ll adjust to your party’s new stance and vote for its nominees. If your philosophical convictions are what matter most, you may break with a party once it no longer upholds them.