Ninety-nine percent of white evangelicals and 98 percent of Republicans who rely on Fox News as their primary news source do not believe Mr. Trump should be impeached and removed, the survey found, compared with 94 percent of Republicans over all.

As committed as those Americans are to Mr. Trump, they alone are not enough for him to win re-election next year. And the survey revealed significant defections among some non-Republican constituencies that helped Mr. Trump win in 2016, namely the white working class. This trend is driven by white women without a college degree, 40 percent of whom now say they support impeaching and removing him, compared with 29 percent who said so in mid-September.

Those figures are especially problematic because of the political clout those women have in the Midwestern states that put Mr. Trump over the top in the Electoral College in 2016. That year, the group supported Mr. Trump over Hillary Clinton, 61 percent to 34 percent.