“They have to come to terms with what they created,” said Laura Ingraham, a conservative activist and talk-radio host. “They’ll talk about everything except the fact that their policies are unpopular.”

The distance was magnified by the Supreme Court’s 2010 decision in the Citizens United case, which gave wealthy donors rising weight in Republican circles, even amid signs that the party’s downscale voters were demanding more of a voice.

Most of these voters had long since given up on an increasingly liberal and cosmopolitan Democratic Party. In Mr. Trump, they found a tribune: a blue-collar billionaire who stood in the lobby of a Manhattan skyscraper bearing his name and pledged to expand Social Security, refuse the money of big donors, sock it to Chinese central bankers and relieve Americans of unfair competition from foreign workers.

The Democratic Party is also reckoning this year with a populist insurgency, driven in part by economic pain and growing anger against Washington and Wall Street. But while Senator Bernie Sanders trails Hillary Clinton in delegates, Mr. Trump’s unlikely campaign has become a seemingly unstoppable force, one that Republican lawmakers, donors and activists are only now fully confronting.

“The Republican Party is being dramatically transformed,” said Foster Friess, a Wyoming investor and philanthropist who is among the party’s most significant donors. Republicans and Democrats alike, Mr. Friess said, had neglected “the people who truly make our country work — the truck drivers, farmers, welders, hospitality workers.”

Seeds of a Split

Six years ago, as the 2010 elections neared, everything seemed to be falling into place.

Republicans celebrated an impending repudiation of President Obama in congressional races, in which they would eventually pick up 63 seats. On the ninth floor of the storied Beresford apartment building on Central Park West, guests clinked glasses at a fund-raiser for Republican Senate candidates hosted by Paul Singer, the billionaire investor.

A self-described Goldwater conservative and proponent of an immigration overhaul deal, Mr. Singer had publicly lamented “indiscriminate attacks by political leaders against anything that moves in the world of finance.” In 2010, Mr. Singer tripled his campaign giving, doling out almost $3 million in contributions to Republicans.