Much ink has been spilled analyzing the civil war within the Republican Party -- the one between the establishment and the Tea Party. While the differences between the two wings are sometimes as much about style as substance, the impact on U.S. Senate primaries has been one of this year’s major political stories.

Six of the 12 Republican Senate incumbents up for re-election faced noteworthy primary competition from their right flank. All of them were backed by the party’s establishment wing, and to GOP leaders’ great relief, each won renomination.

But there was a dog that didn’t bark, and that was that there were hardly any Tea Party challenges to Republican governors this year. All told, 19 Republican governors are up for re-election, but only two -- in Idaho and Wyoming -- faced anything approaching a serious primary challenge from their right, and neither of those challengers defeated the incumbent. That’s just 11 percent of GOP gubernatorial incumbents drawing a primary challenge from the right, compared to fully half of U.S. Senate incumbents. Why?

In a few cases, the incumbent Republican governor escaped a challenge either by being iconic, such as four-term Iowa Gov. Terry Branstad, or widely popular, such as Nevada Gov. Brian Sandoval. Or they represent purple-to-blue states where a Republican further to the right would be a nonstarter with most voters. These states include Pennsylvania, Michigan, New Mexico, Ohio and Wisconsin.

In most of the cases, the governor was already quite conservative to start with, leaving little room to a challenger on his or her right. Governors in this category include Florida’s Rick Scott, Kansas’ Sam Brownback, Maine’s Paul LePage, South Carolina’s Nikki Haley and others. In Kansas, Brownback has “acted both in policy and political terms completely in line with Tea Party goals,” says University of Kansas political scientist Burdett Loomis. “His tax cuts and small government initiatives fit their agenda well,” leaving no room to take him on in a primary.

But perhaps the most fundamental reason that Republican governors haven’t faced primary challenges from their right is that state races are different from federal races. They often turn on different types of issues, such as education and infrastructure, that are more technocratic in nature and aren’t coherent enough to package into a red-meat national agenda. Meanwhile, state political climates tend to vary too much from each other for a national political force to coalesce around them.

Adam Brandon, executive vice president of FreedomWorks, one of the best known groups seeking to organize the Tea Party nationally, says that for many in the movement, it comes down to practicalities. “We have limited resources, and we already know the rules for federal elections,” Brandon says. “I’m not saying we wouldn’t dream to get involved [in gubernatorial races], but you have to walk before you can run.”