The areas of biggest disagreement are climate change, immigration and refugees. Less than 33 percent of young Republicans who approve of Mr. Trump say they view climate change as a serious threat, but among young Republicans who disapprove of the president, that rises to over 60 percent. While over 70 percent of Trump-approving young Republicans believe undocumented immigrants and refugees are a threat to America, 60 percent of Republican Trump disapprovers say the opposite.

If the party continues to bleed younger voters, I believe these policy issues — climate change and immigration will be the drivers. In that case, the party’s turn to Trumpism will have won out among the young who still call themselves Republicans, but at the expense of scaring off many young voters who might have called themselves Republicans in another time. The results from the Alabama Senate race offer a glimpse of this future: In the counties of the University of Alabama and Auburn, support for the Republican, Mr. Moore, plummeted, swinging dramatically away from the margins that Mr. Trump won by in 2016 and toward double-digit wins for the Democrat, Doug Jones.