In conversations with 10 G.O.P. analysts, consultants and activists, all said they were acutely aware of the rising influence of young voters like Mr. Galloway, who in their lifetimes haven’t seen a single month of colder-than-average temperatures globally, and who call climate change a top priority. Those strategists said lawmakers were aware, too, but few were taking action.

“We’re definitely sending a message to younger voters that we don’t care about things that are very important to them,” said Douglas Heye, a former communications director at the Republican National Committee. “This spells certain doom in the long term if there isn’t a plan to admit reality and have legislative prescriptions for it.”

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President Trump has set the tone for Republicans by deriding climate change, using White House resources to undermine science and avoiding even uttering the phrase. Outside of a handful of states such as Florida, where addressing climate change has become more bipartisan, analysts said Republican politicians were unlikely to buck Mr. Trump or even to talk about climate change on the campaign trail at all , except perhaps to criticize Democrats for supporting the Green New Deal.

That, several strategists warned, means the party stands to lose voters to Democrats in 2020 and beyond — a prospect they said was particularly worrisome in swing districts that Republicans must win to recapture a majority in the House of Representatives.

The polling bears out Mr. Heye’s prediction of a backlash. Nearly 60 percent of Republicans between the ages of 23 and 38 say that climate change is having an effect on the United States, and 36 percent believe humans are the cause. That’s about double the numbers of Republicans over age 52.