“You can’t do much more to reduce the overlap between the parties because it’s pretty much gone,” Karol says. “That mostly wasn’t party switching by members. It was mostly turnover, and disproportionately because of Republicans. There’s a lot of evidence that a successive wave of younger, newer Republicans is more conservative than their elders. And you don’t see this going on in a Democratic caucus. White Southern Democrats became far less numerous. But you don’t see in Ohio that the young Democrats are more liberal than the old Democrats.”

For the GOP, that means the era of conservative spoilers isn’t necessarily behind them — conservative groups have a long list of targets for coming election cycles — and for the country it means the polarization and vitriol in Congress will probably intensify.

“The Susan Collinses of the world, the [Mark] Pryors of the world, the [Mary] Landrieus, they could vote with the parties more now,” Karol says. “Or they could lose their seats, and be replaced by orthodox Democrats and Republicans.”

You could call it the Cruz effect. Greater polarization, which in turn feeds toxic behavioral changes — the ongoing breakdown in norms that for the better part of a century helped stabilize legislative governance in the country.