Among the chattering classes, conventional wisdom suggests that President Donald Trump’s standing remains incredibly strong among Republicans. This notion is used to dismiss the possibility that someone (say, a newly disgruntled U.S. ambassador to the United Nations) could muster a serious primary challenge to the president in 2020. It’s also used to absolve congressional Republican enablers of their obsequiousness.

Take, for example, Sen. Bob Corker’s (R-TN) recent comments about Republicans’ reluctance to push back on Trump’s attacks on Robert Mueller. “The president is, as you know—you’ve seen his numbers among the Republican base—it’s very strong. It’s more than strong, it’s tribal in nature,” Corker said. “People who tell me, who are out on trail, say, look, people don’t ask about issues anymore. They don’t care about issues. They want to know if you’re with Trump or not.”

Corker seems to be right about how GOP lawmakers generally perceive the president’s strength within the party. But do the numbers actually affirm this perception?

According to Gallup, Trump’s approval rating among Republicans is currently at 85 percent. This is certainly respectable, but hardly unique. In April 2002, George W. Bush boasted a 98 percent approval rating among Republicans, according to Gallup. This was seven months after the 9/11 attacks but his approval rating among Republicans had been at 87 percent the day of the attacks.

As someone who lived through the Bush era, I can attest that Bush was able to impose pretty strict party loyalty on the right. But by April 2006, his approval rating among Republicans was hovering around 80 percent—not too far from where Trump is now. Those midterm elections were a disaster for the GOP. And it went downhill fast from there.

We tend to remember things like Hurricane Katrina and Abu Ghraib—huge scandals that deservedly hurt Bush with the American public. But, on the right, it was the Harriet Miers debacle that created a permission structure for conservatives to finally begin criticizing a Wilsonian foreign policy, the controversy over the transfer of U.S. ports to a Dubai firm, and, ultimately, to derail Bush’s 2007 attempt at a comprehensive immigration reform proposal that included a pathway to citizenship; or amnesty, for its critics.

The point here is that the danger to Trump isn’t merely that he could be “primaried.” A more likely scenario is that Republican politicians will eventually discover that they can stand up to a Republican president without fear of reprisal. Since fealty to Trump has always been premised on a transactional calculation (as opposed to personal affection, shared goals, or mutual respect), the only thing binding them to Trump is the perception that their political base demands it. When that changes—and history suggests that this happens to even the most popular presidents—the levee breaks.

Sometimes all it takes is one big mistake.

In March 1991, just after the Gulf War ended, President George H.W. Bush’s approval rating with the nation, not just Republicans, stood at 89 percent. Bush boasted a 95 percent approval rating among Republicans his second year in office. (This chart shows second year approval ratings by party going all the way back to Eisenhower.)

And yet, not only did Bush go on to lose the presidency to Bill Clinton in 1992, but he had to fend off tough challenges from populists Pat Buchanan (in the GOP primary) and Ross Perot (who ran a third-party presidential campaign) in the general election.

How did this happen? There is some question as to how important Bush’s breaking of his “Read My Lips, No New Taxes!” pledge hurt him among voters who actually showed up at the polls, but anti-tax crusader Grover Norquist said the reversal “’depressed the hell out of the [Republican] base’ because it was a fundamental breach of faith.’” Clinton certainly contributed to Bush’s woes by running ads ripping the broken promise.

I don’t think that Donald Trump will make this kind of mistake. He is less noble than George H.W., who believed he was doing the right thing for the nation even if it cost him politically. Trump also has a better political instincts when it comes to knowing what a Republican base will tolerate. Still, the defeat of “41” should serve as a cautionary tale for any politician who wants to take solace in their approval ratings.

History suggests it is incredibly difficult to wrest the nomination from a sitting president. Trump is significantly more popular within his party than either Jimmy Carter or Gerald Ford—two presidents who were able to survive primary challenges only to go on to lose the general. The question is not whether Trump could survive a primary if he had to, but how costly it would be. What is more, it is worth examining whether Trump’s popularity with the GOP base justifies the amount of deference some Republican politicians and elites are paying him.

The truth is that there isn’t a huge correlation between the sophomore numbers and political resilience. Both Bushes had sky-high approval numbers their second year in office, and one lost re-election after fending off a primary challenge, while the other narrowly survived re-election, despite avoiding a primary challenge. Meanwhile, Reagan (79 percent) and Nixon (82 percent) both had pretty average numbers their second year among Republicans, and both went on to win huge landslides.

Trump’s popularity with Republicans is really just pretty average. There is little doubt that the intensity among his strongest supporters is high, but this asterisk is overwhelmed by another important caveat. As Gallup notes: “ Fewer Americans identify as Republicans or say they are Republican-leaning independents than did so in November 2016, the month Donald Trump was elected president.”

It may be that Trump is popular among people who identify as Republicans, simply because the Republicans who don’t like him are… no longer Republicans. In others words: Trump’s approval rating in his party climbs because his party is shrinking. Maybe Bob Corker shouldn’t be quite so afraid.