Over the last few decades, residents of many traditionally liberal states have moved to states that were once more conservative. And this pattern has played an important role in helping the Democratic Party win the last two presidential elections and four of the last six. The growth of the Latino population and the social liberalism of the millennial generation may receive more attention, but the growing diaspora of blue-state America matters as well.

The blue diaspora has helped offset the fact that many of the nation’s fastest-growing states are traditionally Republican. You can think of it as a kind of race: Population growth in these Republican states is reducing the share of the Electoral College held by traditionally Democratic states. But Democratic migration has been fast enough, so far, to allow the party to overcome the fact that the Northeast and industrial Midwest contain a smaller portion of the country’s population than they once did.