Many once-Republican young people left the party, and many young people who might have considered themselves Republicans in the past are simply electing to stay independent. In the most recent Harvard Institute of Politics poll of Americans under 30, only 22 percent identified as Republican. The largest affiliation category was independent, at 39 percent. Research by NBC has confirmed that young voters increasingly see no need to identify with either party and are unsure if either one — but again, especially the Republican Party — cares about them. Only 6 percent of millennials in the NBC study said that they strongly approved of the job President Trump is doing (and 46 percent strongly disapproved).

In the Trump era, young voters may be walking away from the parties themselves, but they are voting quite like Democrats. (The Virginia governor’s race is an excellent example.)

Many Republicans are not terribly concerned about this issue. They point to strong support for Mr. Trump among young Republicans to suggest that all is well, ignoring that the remaining young Republicans are light-years away from their peers these days in terms of political attitudes.

It is true that young Republicans who have stayed in the party are mostly in favor of the president: The Harvard poll found that 66 percent of them approved of the job Mr. Trump was doing. That’s lower than the president’s approval among Republicans over all, but it’s still a significant majority.

But a deeper dive into the Harvard poll revealed three areas of disagreement between those who approve and disapprove of Mr. Trump as well as young independents who disapprove of Mr. Trump but nonetheless say they would prefer a Republican Congress.

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.