Colorado is turning purple because of migration.
There have been other, longer explanations for why traditionally Republican Colorado is in play this election. Real Clear Politics has all the numbers. Ryan Lizza in the New Yorker has a good article on the ascendance of Democratic politicians in the state. Christopher Beam does the same fine job of parsing the politics of the state on Slate .
But migration tells a better story.
Colorado hasn't become what Stuart Rothenberg calls the most important state in November because of what politicians have or have not done or because of a new array of issues. Colorado has been trending Democratic because the people who have moved there largely come from Democratic counties in other states.
Colorado has imported Democrats.
Bob Cushing tracked the flow of people in and out of Colorado from 1981 to 2005 using migration data collected by the federal government. Some Colorado counties had a lot of flux in their populations. Others were relatively stable.
Cushing divided the state's 63 counties into three even groups based on migration. (OK, Colorado has 64 counties, but Broomfield is relatively new.) The 21 counties with the highest percentage of new population were the most Democratic in the 2004 election. They voted two percentage points more Democratic than the state as a whole. (These were the densely populated counties around Denver and Boulder.)
The middle group was eight percentage points more Republican than the Colorado average. And the last group of counties—those whose populations have been least affected by out-of-towners—voted 15 points more Republican than the state average.
The larger the number of newcomers in a Colorado county, the more Democratic that county voted in 2004.
This change has been taking place slowly, just like migration. But examined over a generation, the politics of the migration have shifted dramatically. The vote for the presidential elections from 1992 to 2004 in these 21 counties was 19 percentage points more Democratic than in the period from 1976 through 1988.
The in-between counties have grown 10 percentage points more Democratic.
The 23 Colorado counties least affected by the outside world have grown one percentage point more Republican.
And, yes, as expected, the people moving into the counties growing most Democratic come from counties that voted blue in presidential elections. The county outside of Colorado that sent the most people to the state over the last generation was deeply Democratic: Los Angeles.
When people move, they sort by political preference. It's happening all over the country. The new people moving into Northern Virginia are turning that area Democratic. Meanwhile, the county clerk in Crook County in rural central Oregon said that eight out of 10 people who registered with a party there since 1995 have been Republican.
Every four years there's a presidential election. But Americans vote with their feet every day.