In the command centers of Republican presidential campaigns, aides have drawn comfort from the belief that Donald J. Trump’s dominance in the polls is a political summer fling, like Herman Cain in 2011 — an unsustainable boomlet dependent on megawatt celebrity, narrow appeal and unreliable surveys of Americans with a spotty record of actually voting in primaries.

A growing body of evidence suggests that may be wishful thinking.

A review of public polling, extensive interviews with a host of his supporters in two states and a new private survey that tracks voting records all point to the conclusion that Mr. Trump has built a broad, demographically and ideologically diverse coalition, constructed around personality, not substance, that bridges demographic and political divides. In doing so, he has effectively insulated himself from the consequences of startling statements that might instantly doom rival candidates.

In poll after poll of Republicans, Mr. Trump leads among women, despite having used terms like “fat pigs” and “disgusting animals” to denigrate some of them. He leads among evangelical Christians, despite saying he had never had a reason to ask God for forgiveness. He leads among moderates and college-educated voters, despite a populist and anti-immigrant message thought to resonate most with conservatives and less-affluent voters. He leads among the most frequent, likely voters, even though his appeal is greatest among those with little history of voting.

The unusual character of Mr. Trump’s coalition by no means guarantees his campaign will survive until next year’s primaries, let alone beyond. The diversity of his coalition could even be its undoing, if his previous support for liberal policies and donations to Democrats, for example, undermine his support among conservatives. And in the end, the polling suggests, Mr. Trump will run into a wall: Most Republicans do not support his candidacy and seem unlikely ever to do so. Even now, more say they definitely would not vote for him than say they support him.