Blindsided by the re-election of President Barack Obama, the Republican National Committee famously conducted an “autopsy” of the 2012 election to identify what went wrong. The resulting 100-page “Growth and Opportunity Project” report ultimately blamed Mitt Romney’s loss on the G.O.P.’s failure to connect with women and minority voters, particularly Latinos, and recommended that Republicans make comprehensive immigration reform a priority. But the effort to engineer a kindler, gentler G.O.P. backfired: the party’s conservative base punished lawmakers who strove to work across the aisle (just ask chastened “Gang of Eight” member Marco Rubio) while elevating extremists like Donald Trump, who promised to impose harsh, new anti-immigration policies.

Seeing which way the wind was blowing, R.N.C. chairman Reince Priebus effectively agreed to throw his own post-2012 playbook out the window, committing the national party to doubling down on the white vote. But recent events have eroded Trump’s core constituency. Following weeks in which Trump was accused by nearly a dozen women of sexual harassment or assault, the Republican nominee has hemorrhaged support among the same white voters who fueled his rise. Even white, working-class men—the most over-scrutinized, fetishized voting bloc of the 2016 election—have seemingly had enough of Mr. Trump.

At the end of September, prior to the first presidential debate, the real-estate mogul was closing in on Clinton in the polls. According to an ABC News poll taken at the time, Trump trailed the former secretary of state by a mere two points (44 percent to 46 percent) nationally; among white men, the G.O.P. nominee held a commanding 40-point lead. But in one month’s time, that edge has narrowed to 17 percentage points, according to the latest ABC News poll. That’s the same percentage that Romney received in pre-election polls in 2012—and Romney lost. Trump’s support has even slipped with non-college-educated white-male voters, falling from 76 percent at the end of last month to 60 percent in the most recent poll, which was released on Sunday. Over the same period, Clinton’s support among white men surged by 10 percentage points, with a 12-point increase among white men without a college degree.

White women, unsurprisingly, have also abandoned Trump. At the end of September, Clinton was polling among white women at 46 percent; in the most recent ABC News poll, she leads Trump 50 to 43. Comparatively, among white women with and without college degrees, Clinton rose five points to 62 percent, and two points to 42 percent, respectively.

Overall, Trump is still leading slightly among whites, 47 to 43. But his dramatic losses among his core supporter base illustrate that there is a limit to how far divisive rhetoric can carry a candidate before it becomes a liability. Last year, Republicans aligning themselves with Trump bet that his ability to galvanize the white vote might lead to a surge in new voters, expanding the G.O.P. and allowing the party to triumph over the younger, more diverse “Obama coalition.” But as James Carville noted in a recent interview with Vanity Fair, that’s always been a losing strategy in the long-term:

“The demographic changes in the country are substantial, and they are changing rapidly in a way that is unfavorable to the Republican Party. And the Republicans, I would say, continue to make a bad bet. They keep doubling down on these non-college whites. This has proven to be very successful in certain parts of the country and in off-year elections, but it has not worked very well at all in presidential years. It’s working terribly now.”

Priebus and other Republican Party leaders, mesmerized by the surge of support for Donald Trump among white men in the primary race, hoped that they might still win a national election with the G.O.P.’s traditional base alone, regardless of the conclusions of their own 2012 autopsy report. Now, that decision appears to be leading them straight to another resounding defeat. You can’t say they didn’t try to warn themselves.