At Sabato’s Crystal Ball, Kyle Kondik runs some numbers about the relative vulnerability of each party’s incumbent senators, going back to the end of WW2. And he discovers something interesting: Democrats seem to be significantly better than Republicans in flipping Senate seats held by incumbents.

That superiority is obscured, notes Kondik, by a single election, in 1980, when the GOP beat nine Democratic incumbent senators in November and won three other seats where the incumbent lost in a primary. Since then, such successes have been few and far between:

Incredibly, in the 16 Senate elections since then, the Republicans have flipped only 12 Democratic Senate seats where the incumbent was running again: It’s taken them three decades worth of elections to match the achievement of that single 1980 effort….

[A]fter 1980: Democrats have flipped more than triple the number of Republican seats where an incumbent was running for reelection, 38 for them versus just 12 for the Republicans. That includes two very big years for Democrats where they took control of the Senate largely through the defeat of incumbents: They won seven seats in states where an incumbent Republican ran for reelection in 1986 to flip the chamber, and then netted their six-seat gain and slim 51-49 majority in 2006 by exclusively beating incumbents.

This analysis brings back memories of the 1980 election night, when, incredibly, every single close Senate race, and there were a ton of them, fell to the GOP, taking down a lot of big names, such as McGovern, Church, Bayh, Culver, Magnuson, Nelson, and Talmadge.

But it doesn’t explain why, with this one exception, Republicans have been feckless in taking on incumbents since then, unless they used up wishes to some genie.

Kondik calls this record a “hidden barrier” to a Republican takeover of the Senate this November. I’m not sure that makes any sense without some explanation of why the R label would be a handicap in individual races that sure look winnable at this point. But the numbers are truly striking, and ought to at least create some sweaty nights of concern for NRSC ops this summer.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.