IN ANTICIPATION OF TOMORROW’S SPIN…. Tomorrow should be a pretty interesting day for primary campaigns. Chris Cillizza has already labeled it “one of the most important voting days of the year.”

That’s only slightly hyperbolic. Pennsylvania’s Senate Democratic primary, Arkansas’ Senate Democratic primary, both parties’ Senate primaries in Kentucky, and two House special elections will offer some strong hints about public attitudes, anti-incumbent trends, prevailing winds, and the position the parties occupy in advance of the midterms.

But we can probably guess how the media will perceive the results, in large part because the message is already being disseminated. Dana Milbank has described the Democratic primaries, for example, as “purity putsches.” The Washington Post editorial board used the primaries to lament the “ideological purification of both parties.”

It was good to see E.J. Dionne Jr. knock the silly effort to draw parallels between recent developments in both parties.

This year’s elections may exacerbate the difference between our two political parties, but not in the way most people are talking about.

With incumbent Democratic senators under threat in two more primaries on Tuesday, the conventional view is that Republicans and Democrats will emerge from this election more ideologically polarized than ever. Primaries will push Republicans to the right and Democrats to the left.

That’s only half true. Republicans will, indeed, end the year a more philosophically coherent right-wing party. But the Democrats will, if anything, become more ideologically diverse.

Imagine that. For all the assumptions about and “party purges,” Dems aren’t “purifying” the party along ideological lines at all. Rep. Alan Mollohan (D-W.Va.) lost his primary to a challenger from the right. Mark Critz is poised to do fairly well tomorrow in Pennsylvania’s 12th despite running away from the progressive agenda. Sen. Arlen Specter (D-Pa.) has been pretty reliably progressive on legislative matters in recent months, but he may very well lose tomorrow.

“[A]ll this underscores the real difference between the two parties,” Dionne noted. “The Democrats will remain an intricate coalition that struggles to hold together the left, the center and bits of the right. Republicans, as Arlen Specter could tell you, are the ones opting for ideological purity.”

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.