Tuesday's statewide primary in Illinois set up a potentially close contest in November, with incumbent Democratic Gov. Pat Quinn facing Republican Bruce Rauner. But Illinois' a blue state, right? Walk in the park for Quinn? Not this time. And, in fact, not often in recent years.

The two candidates are very different. One columnist for the Chicago Tribune predicts that the race will become a "class war," with Rauner, a venture capitalist, battling against both Quinn and the state's powerful public sector unions. Only two polls have pitted the two against each other: each candidate leads in one. A Republican governor in Illinois. Who'da thunk it?

People in 2003, for one. As Slate's Dave Weigel noted on Twitter on Tuesday, "a generation has grown up knowing IL as a safe D state." But: "Rs held the governor's office for decades." That's true of two other solidly blue states, too: New York and California are only recently solid blue states. In part that's because the parties have evolved over the years; in part it's because more liberal Republicans tend to do better in statewide elections in blue states. But Weigel's point stands: Democratic victory in any of the three states is not guaranteed.

Curious about how that had evolved over time, I pulled data on the states' governors since 1900. And, in fact, only 36 percent of the past 114 years have any of the three states had a Democratic governor. While all three have Democratic governors now, that's only been the case since 2011, when Arnold Schwarzenegger left office in California. Since 1899, there have only been seven years in which Democrats controlled the governor's seat in all three states.

The breakdown by state is as follows:

Illinois

California

New York

Only California has had a Democratic governor for the majority of the time since 1900 — and that, only barely.

We took it one step further, plotting each governor's tenure over time. (California's data goes to January 1899, a year prior to the other two states, so its bar is a little longer.)

The moral of the story is a simple one. Illinois has been a blue state for only about a third of the past century. Pat Quinn isn't any safer there simply because he's a Democrat than he would be in, say, Ohio. "Blue state" is ephemeral.