Nate Silver: Registered Voter Polls Will (Usually) Overrate Democrats

538 reports:

Over the long run, Republicans and Democrats each win elections about half the time. This is not only a strong historical tendency1, but also one well predicted by political theory. A party that continually loses elections because it’s too far removed from the political center or because it appeals to too narrow a range of voters should seek to remedy that by expanding its coalition or moving toward the median voter — perhaps by compromising some of its ideological goals.

Sometimes the transition can be rough. The Republican Party, for example, is currently engaged in a battle between its more moderate and more conservative forces. It’s not clear the conflict between the Republican establishment and the so-called tea party is over. Nonetheless, parties usually become more moderate before long. In presidential elections, they’ve tended to nominate successively more moderate candidates the longer they’ve been out of the White House. Democrats, for instance, chose Bill Clinton in 1992 after losing with more liberal nominees in 1984 and 1988. And Republicans nominated more moderate candidates in Senate races this year than they did in 2010 or 2012.

But that the parties tend toward electoral parity doesn’t mean their coalitions are mirror images of each other. For a long time in American politics, Republicans have represented those voters who are in the majority (or plurality) demographically. A voter who is white, straight, suburban, Christian, middle-aged and middle income is quite likely to be a Republican.

An article well worth your time.