Read: Trump’s entire shutdown approach, encapsulated in one tweet

For months, Trump’s overall approval rating has been an object of fascination for pundits. On the one hand, it’s terrible, hovering in the low 40s, as HuffPost Pollster’s average demonstrates. On the other hand, it has just hovered there. Almost nothing—not the steady drumbeat of damning news on the Russia investigation, not the chaos of the White House, and neither a strong economy nor a volatile stock market—seemed able to dislodge it. The American people had apparently made up their minds about Trump, and the four in 10 who approved weren’t going to change their minds, come hell or high water.

Yet the shutdown seems to have broken that equilibrium. Trump hit his highest disapproval on record in the Morning Consult poll, at 57 percent. CBS found 59 percent disapproval. In the AP-NORC poll, Trump’s approval tanked from 42 percent a month ago to 34 percent. The president was even down in an NPR/PBS NewsHour/Marist poll he flogged on Twitter. FiveThirtyEight’s poll aggregator shows a clear downward trend since the start of the shutdown, with Trump’s approval heading toward depths not seen since his disastrous December 2017 and the aftermath of a white-supremacist march in Charlottesville, Virginia, in August 2017.

What’s interesting is not just that the approval rate is finally budging, but why—and with whom. Trump has long been happy to withstand the opprobrium of the press, elites, and much of the country. As my colleague Ron Brownstein has demonstrated, the president has opted for political tactics meant to shore up his base. This is strategically dubious—it’s hard to win reelection with 40 percent of the country—but it has been consistent. It has shown some success, too. While Trump’s overall approval is low, his standing among Republicans has remained very strong.

Shutting down the government over the wall was a part of that philosophy. The administration concluded that the wall was such an important issue for his base that it was worth whatever political blowback that might come from other quarters to get it done. As Ann Coulter, who effectively shamed him into the shutdown in December, recently put it, “He is dead in the water if he doesn’t build that wall. Dead, dead, dead.”

In practice, however, weakened standing among that base accounts for Trump’s slumping approval. The NPR/NewsHour/Marist poll finds that Trump’s approval is down among suburban men, white evangelicals, and men without a college degree, all key segments of his constituency. And while 83 percent of Republicans in that poll still support him, his net approval has slipped 10 points.

Read: Why hasn’t Trump folded?

In the Politico/Morning Consult poll, Trump’s disapproval increased among evangelicals, non-college-educated voters, and those who voted for Trump in 2016, compared with a poll in early January. Meanwhile, the number who blamed Trump for the shutdown increased (slightly) among all three groups.