New data from YouGov and Public Policy Polling show the extent to which he has tapped into a set of deeply rooted racial attitudes. But first, two caveats about these data are worth bearing in mind. The national YouGov survey was done near the middle of January, before the Iowa caucuses and New Hampshire and South Carolina primaries. Public Policy Polling is a company aligned with the Democratic Party, and some of its results over the years have been suspected of bias. Taken by itself, its conclusions could be doubted. Taken with the YouGov and exit poll data, however, these three surveys can give us a better idea of Mr. Trump’s backers.

Mr. Trump’s support among those who say they support a temporary ban on Muslim entry into the United States — a notion Mr. Trump first advanced in early December — is significant. He won more than twice as many supporters of the ban in South Carolina as any other candidate. Voters often echo the things candidates say on the campaign trail, so that level may not be revelatory.

Possibly more surprising are the attitudes of Mr. Trump’s supporters on things that he has not talked very much about on the campaign trail. He has said nothing about a ban on gays in the United States, the outcome of the Civil War or white supremacy. Yet on all of these topics, Mr. Trump’s supporters appear to stand out from the rest of Republican primary voters.

Data from Public Policy Polling show that a third of Mr. Trump’s backers in South Carolina support barring gays and lesbians from entering the country. This is nearly twice the support for this idea (17 percent) among Ted Cruz’s and Marco Rubio’s voters and nearly five times the support of John Kasich’s and Ben Carson’s supporters (7 percent).