The failure to construct a credible general election fund-raising and field organization eminently justified Donald Trump’s decision this week to fire his combative campaign manager Corey Lewandowski. But it’s likely that Trump himself has already made the decisions that will most shape—and constrict—his general-election prospects.

As a first-time candidate with no record in public office, the most important decision Trump faced was how to define himself to the public. From the outset, he has stressed three principal identities. One is as a savvy business executive who would use his private-sector smarts to turn around the government and economy. The second is as a political outsider untethered to special interests who will clean up a self-serving political system. But through the primaries he subordinated each of those to a third emphasis: his role as the embodiment of resistance to America’s rapid demographic and cultural change.

A major national poll released Thursday morning illuminates how strong a tailwind that definition provided to Trump during the Republican primary—and how fierce a headwind it presents for him in a general election. The poll also helps clarify why the contest between Trump and Hillary Clinton is likely to pivot more on questions of national identity—how we live together, or not, in a rapidly diversifying country—than on any other issue.

The survey, by the non-partisan Public Religion Research Institute, and the center-left Brookings Institution, measures Americans’ attitudes about a broad range of issues relating to immigration and demographic change. Consistently, the poll found that Trump supporters view the changes with greater—often much greater—alarm than not only Democrats or independents, but also Republicans who did not support Trump during the GOP primaries. In all, the survey shows that Trump was lifted by a coalition that largely believes the America it has known is under siege—and that unprecedented measures are required to reverse the threat.

According to figures provided to me by PRRI, Trump supporters (including both Republicans and GOP-leaning independents who backed him during the primary) are more likely than Democrats, independents or other Republicans to say that they worry about being a victim of terrorism or violent crime; that they are bothered when they hear immigrants talking in a language other than English; that discrimination against whites is as great a problem as discrimination against minorities; and that American and Islamic values are inherently at odds. Fully 80 percent of Trump voters say that immigrants are more burden than benefit to America; just 27 percent of Democrats, 41 percent of independents, and 53 percent of other Republicans agree.

Often the contrast between Trump supporters and all other adults widened further when the poll measured those who hold these positions most vehemently. Fully 44 percent of Trump supporters, for instance, said they “completely agree” it bothers them when they hear immigrants speaking a language other than English; less than half as many independents, Democrats, or non-Trump-supporting Republicans agreed. Likewise, while about two-fifths of Trump Republicans “completely” agreed that “because things have gotten so far off track in this country, we need a leader who is willing to break some rules,” less than one-fifth of Democrats, independents, and other Republicans concurred.

That instinct helps explain the broad support in Trump’s coalition for his edgiest proposals; indeed, the poll makes clear that Trump triumphed not in spite of his most polarizing ideas, but largely because of them. Roughly four-fifths of Trump supporters say they back his plans to build a wall with Mexico, to temporarily ban all Muslims from entering the country, and to bar Syrian refugees. In each case, between 43 and 47 percent of Trump supporters back those ideas strongly.

But each of those three ideas face solid majority opposition from the public overall, drawing support from only about one-quarter of Democrats and two-fifths of independents. (Most non-Trump-supporting Republicans do back them, but by much narrower margins.) Similarly, the poll found, less than one-quarter of independents, Democrats, or other Republicans support Trump’s pledge to deport all undocumented immigrants; even most of his own backers say they would allow the undocumented to obtain legal status and remain.

The poll shows that Trump’s insular message may find its biggest audience at the intersection of immigration and terror; most respondents said they considered Islamic and American values at odds. After the Orlando massacre, that sentiment could rise further. And the poll also shows anxiety about the impact of immigration on wages.

But the survey’s overall message is that Trump’s emergence represents a triumph for the most ardent elements in the GOP’s “coalition of restoration,” voters who are resistant to demographic change. Barring an unlikely convention coup, their success in propelling him to the nomination has put the party at odds not only with America’s growing minority population, which is expressing stratospherically high unfavorable sentiments about Trump in surveys. It’s also alienated many college-educated whites (especially women), who are much more likely than their non-college-educated counterparts to recoil from Trump’s racially-tinged nationalism. No Democratic presidential candidate since 1952 has carried most college-educated whites, but two national surveys released this week have showed Hillary Clinton leading Trump among them by at least six percentage points.

Those results capture the limits of this week’s abrupt mid-course correction.  However the candidate retools his general election strategy after dismissing Lewandowski, the choices Trump made to win the primary have already left him with only a narrow and precarious path to the White House.