These commentators and candidates look backward to an America that is being lost. Ann Coulter encapsulated this attitude perfectly in her latest book title, “Adios, America.” This is the philosophy of the receding roar, the mourning for an America that once was and is now being destroyed by foreign people and ideas.

Out of this backward- and inward-looking mentality comes a desire to exclude. Donald Trump talks falsely and harshly about Hispanic immigrants. Ben Carson says he couldn’t advocate putting “a Muslim in charge of this nation.”

During George W. Bush’s first term there wasn’t much difference between how Democrats and Republicans viewed the overall immigration levels. Republicans were about eight percentage points more likely to be dissatisfied with the contemporary immigration flows. But now the gap is an astounding 40 percentage points. Eighty-four percent of Republicans and 44 percent of Democrats are dissatisfied with the current immigration level, according to Gallup surveys.

As Peter Wehner, a longtime conservative writer who served in the Bush administration, wrote in the magazine Commentary: “The message being sent to voters is this: The Republican Party is led by people who are profoundly uncomfortable with the changing (and inevitable) demographic nature of our nation. The G.O.P. is longing to return to the past and is fearful of the future. It is a party that is characterized by resentments and grievances, by distress and dismay, by the belief that America is irredeemably corrupt and past the point of no return. ‘The American dream is dead,’ in the emphatic words of Mr. Trump.”

It’s not exactly breaking news that this is ruinous to the long-term political prospects of the party. In his book “2016 and Beyond,” the veteran pollster Whit Ayres, now working for Marco Rubio, points out that given the composition of the electorate, if the G.O.P. candidate won the same 59 percent share of the white vote that Mitt Romney won in 2012, he would have to win 30 percent of the nonwhite vote to get a majority. That’s a daunting number, given that, as Dan Balz of The Washington Post points out, Romney only won 17 percent of that vote.