Writing at the Wall Street Journal earlier this month, Obama advisor David Plouffe made a prediction: "States like Georgia, Arizona and yes, even Texas, will be purple states very soon. Good for the Democrats." It would be good for the Democrats. But saying those states will be in play "very soon" is overly optimistic.

Earlier this week, our colleagues over at Monkey Cage made a prediction that Georgia wasn't going to turn blue in the near future. To demonstrate this point, they looked at a very specific bit of data: The relationship between how strongly the state supported Republican candidates versus how strongly the rest of the country did. That margin wasn't getting much narrower, suggesting that the state was remaining pretty red.

This is an interesting concept, and one that could use some more explanation. Think of 2008. In 2008, Barack Obama won the presidency by a large margin — over 7 percentage points nationally. In Pennsylvania, he won by even more, 10.3 percent. But compare that to Indiana, which backed Obama by just over 1 percentage point. Indiana voted for Obama — but was more Republican than the nation on average. Pennsylvania was more Democratic.

We evaluated those margins for each state for the last century. The darker the red on the map below, and the more Republican the state was in relation to the country overall in that election. The darker the blue, the more Democratic.

There are a few weird years, usually when third-party candidates were on the ballot (like George Wallace in 1968). But in general, watching a state shows how it has evolved over time. We pulled out four and graphed them separately.

Texas and Alabama went from much more Democratic to much more Republican (with two weird years, including 1964 when the state's Democrats protested Lyndon Johnson's nomination). But the trend is clear.

Now here's what Monkey Cage was getting at: See how Virginia has slowly moved back toward Democratic candidates over the last few presidential elections? Well, Georgia hasn't. (We added North Carolina as an interesting intermediary state — which, we'll note, went for Obama in 2008.)

Georgia remains to the right of the rest of the country, and isn't moving very much toward the center.

And there's one last interesting point. We took the average of the more Democratic and more Republican states in each election to see if any trends emerge. And one did. Before the South flipped from Democratic to Republican, the margin for states that voted more Democratic was higher than those that voted more Republican. There's a link to population in this, of course: Smaller states can be farther from the midpoint without affecting the national average. In recent elections, it's the Republican states that have been farther from the midpoint — and increasingly so.