In all, no recent survey has found that more than 43 percent of Americans support the wall. That suggests that, as with his overall job performance, Trump has made virtually no progress in broadening his audience since his election: In the exit poll on Election Day 2016, 41 percent of voters said they supported a border wall.

The shutdown is even less popular: In a PBS/Marist poll released Wednesday, 70 percent rejected closing the government to advance a particular policy goal, as Trump has. And by a consistent margin of about 25 percentage points, more Americans blame Trump than congressional Democrats for the impasse, according to roughly a half-dozen recent surveys. Trump’s overall approval rating has fallen to 37 percent in the latest surveys from CNN, Gallup, and Pew, and registered slightly above that in Quinnipiac’s poll.

The Democratic pollster Mark Mellman told me that throughout American history, it hasn’t been unusual for the political system to bottle up policies that most Americans support. One modern example is universal background checks for gun sales. Less typical, he said, is for political leaders to insist on driving through an idea, such as the border wall, that most Americans have clearly indicated they reject. “It’s mind-boggling to me that the whole Republican Party, with a couple of exceptions, has gone along with closing the government to spend money on something most people oppose,” he said.

The most striking aspect of the shutdown might not be Trump’s indifference to majority opinion: He’s demonstrated over and over that he is comfortable playing on the short side of the field so long as his core supporters are energized. Instead, this confrontation might be remembered as the moment that crystallized how much of the Republican Party shares his disregard about appealing to a national majority. As Mellman noted, only a handful of Republicans in either chamber have broken from Trump’s shutdown strategy so far, and even they have dissented only gently.

That hesitance might reflect several factors, including Republicans’ fear of generating a primary challenge by challenging Trump. But even more important might be the extent to which the GOP is now sheltered from the implications of majority opinion. Especially under Trump, the Republican Party is folding in on itself. It is growing more and more dependent on its core supporters and more reliant in both the House and Senate on strongly Republican areas where those voters predominate.

Just three House Republicans (Brian Fitzpatrick in Pennsylvania, John Katko in New York, and Will Hurd in Texas) represent districts that voted for Hillary Clinton in 2016. Even more strikingly, just two House Republicans (Mario Diaz-Balart in Florida and Don Bacon in Nebraska) are left in districts that Trump carried by fewer than 5 percentage points, according to figures provided by TargetSmart, a Democratic voter-targeting firm. After the party’s suburban wipeout in November, more than 85 percent of House Republicans now represent districts that are whiter than the national average, and more than 75 percent hold seats with fewer college graduates than average, according to census figures.