Republican front-runner Donald Trump says he can't wait to take on Democratic front-runner Hillary Clinton in the fall, but here are three reasons why he could lose a general election. (Sarah Parnass/The Washington Post)

If Donald Trump secures the Republican presidential nomination, he would start the general election campaign as the least-popular candidate to represent either party in modern times.

Three-quarters of women view him unfavorably. So do nearly two-thirds of independents, 80 percent of young adults, 85 percent of Hispanics and nearly half of Republicans and Republican-leaning independents.

Those findings, tallied from Washington Post-ABC News polling, fuel Trump’s overall 67 percent unfavorable rating — making Trump more disliked than any major-party nominee in the 32 years the survey has been tracking candidates.

Head-to-head matchups show Hillary Clinton, as well as her Democratic rival Bernie Sanders, leading Trump, often by double digits. Even his two remaining fellow GOP contenders this week backed away from earlier promises to support the eventual nominee.

And with each passing day, Trump makes moves that add further uncertainty to his ability to pivot to the general election. His defiant defense this week of his campaign manager, Corey Lewandowski, who was charged with battery for yanking the arm of a female reporter, as well as Trump’s remarks Wednesday that women who get illegal abortions should be punished, might play well with his followers, but could further alienate the broader electorate.

“Normally, when you’re in a hole, the best advice is to stop digging. That doesn’t appear to be his inclination,” GOP strategist David Carney said. “It’s like taking a wagon full of nitroglycerine across the prairie. It’s great if you get to the mountains and blow them up for gold. But it’s pretty unpredictable.”

Peter Hart, a veteran Democratic pollster who has studied public impressions of Trump, said voters’ views of him are “exceptionally rancid.”

“In terms of any domestic personality that we have measured, we’ve never seen an individual with a higher negative,” Hart said.

Trump has drawn huge crowds and built a passionate base of supporters who have helped him amass a big delegate lead in the battle for the nomination.

But his success among a segment of the Republican electorate stands in contrast to his weaknesses in a general election decided by all voters.

In that broader context, his dismal standing by all traditional measures points to a big question underlying his nontraditional candidacy: whether Trump, as the GOP nominee, could leverage his celebrity persona and unusual appeal among disaffected voters in both parties to overcome his glaring disadvantages.

Trump’s unpopularity in the Post-ABC poll was driven in part by sharply negative ratings from Democrats and lukewarm Republicans. The greatest risk for his general election viability stems from the unusually poor ratings he gets from swing-voting independents and white college graduates.

A silver lining for Trump is that voters overall also feel antipathy for Clinton, the Democratic front-runner. The distaste for Clinton is not as strong as it is for Trump — 52 percent of voters see her unfavorably — but Clinton’s vulnerabilities, combined with Trump’s unpredictability, haunt many Democrats.

Guy Cecil, chief strategist for the pro-Clinton super PAC Priorities USA, urged Democrats to “postpone the ticker-tape parade,” warning that Trump is not as weak a general election candidate as the current atmosphere would suggest.

“I am skeptical of the polls showing such large leads, and it’s incumbent upon us to view this as a close race,” Cecil said. “He’s going to attempt to throw everything, including the kitchen sink and maybe the refrigerator and stove, at Hillary. And I would not be surprised if he changes his views on policy issues.”

Overcoming his hurdles likely would require either a massive influx of working-class white male voters — Trump’s base — or dramatic changes in his policies and presentation that might reverse the strongly negative views of him held by women and minorities.

Trump and his advisers say they have plans to accomplish both objectives. They say he can reverse his favorability ratings over time by framing the fall contest around issues on which they think Trump’s positions resonate powerfully across traditional demographics: the economy, trade and national security.

Since Trump is not tethered to any particular ideology, his test may be convincing voters that he is not a hostile force and is fit to be president, rather than persuading them to buy into a sweeping conservative ideological project.

The Trump team insists that the power of his personality and the potency of his planned attacks on Clinton would win him converts. And it is wagering that millions of working-class voters who for a generation have been politically dormant will rush to the polls and offset Trump’s sizable deficit with the ascendant electorate of women, minority and young voters.

“What you’ll find is across the board, in states like Pennsylvania or New York or New Jersey or Michigan, you’re going to have a bunch of blue-collar workers who have supported Trump in the past and will continue to do so,” Lewandowski said. “That broad appeal allows him to expand the electoral map.”

Concerned about his standing in the polls, Trump’s allies are offering advice about how to make up ground with important demographic groups.

Newt Gingrich, a former Republican House speaker who is unaffiliated but has informally counseled Trump on several occasions, suggested he campaign in black neighborhoods, send targeted messages on social media and embrace his outsider approach to government.

“Imagine Trump on the South Side of Chicago saying, ‘People shouldn’t be killed, schools ought to actually work, you ought to have jobs in your neighborhood and you know that Hillary can’t deliver any of those because she is the system,’ ” Gingrich said.

The shift from a primary fight to the general campaign would be Trump’s crucible, requiring him to communicate persuasively with an entirely different electorate than the primary voters he has courted for the past year.

Ben Carson, the retired neurosurgeon who endorsed Trump after dropping out of the Republican presidential race, said he has advised Trump to turn his attention to education reform and charter schools as a means of supplementing his core pitch on trade and immigration to grow his support with young and minority voters.

“Creating ladders of opportunity, such as school choice, is one way to do that,” Carson said. “He’s been very enthusiastic about that suggestion. He’ll have to follow through and get through to those kids and families who don’t feel like they’re getting the best possible education.”

There are stylistic changes Trump can make, as well, Carson said. “A little humility would go a tremendous distance, no question about it,” he said. “Hopefully, he will find that on his own.”

Frank Luntz, an unaligned GOP pollster, said Trump could erase at least some his deficit if he capitalizes on the fall debates and other events, noting that history is littered with examples of candidates doing just that.

“The big moments cause people to change,” Luntz said. “And let’s face it, we may have a moment outside of conventions and debates that’s even bigger. If you have a Paris or a Brussels on American soil, that can completely change the dynamic.”

It is a tall order, however, for Trump to undo the damage his rhetoric has already done to his image with the rising national electorate that includes Latinos, single mothers and millennials.

“Donald Trump’s whole message is somewhat backward looking,” said Kristen Soltis Anderson, a Republican pollster who wrote a book, “The Selfie Vote,” about these voters.

Referring to Trump’s slogan, she added: “ ‘Make America Great Again’ sounds like an attempt to turn back the clock to a time most young voters don’t remember.”

Pennsylvania, a Democrat-leaning battleground that Trump hopes to target, is a case study of Trump’s upside and downside. While he has picked up endorsements and blue-collar support in the state’s industrial regions, centrist Republicans from Philadelphia and its vote-rich suburbs have kept their distance. Trump needs to make inroads to win a state Republicans last carried in 1988.

“Ticket-splitting Republicans in the Philadelphia suburbs went for [President] Obama — and if they don’t feel comfortable with Trump, they could go for Clinton,” said G. Terry Madonna, a professor at Franklin & Marshall College, which conducts polling in Pennsylvania.

Madonna said that more than 120,000 voters statewide, mostly Democrats and independents, have switched their registration to Republican since January. But he cautioned against interpreting the moves as a Rust Belt tilt toward Trump.

“Even if these children of Reagan Democrats love his talk about manufacturing and American pride, he’s going to have to make sure he’s not losing the Republicans who are the heart of the party,” said John Brabender, a GOP strategist who has guided the political career of former senator Rick Santorum (R-Pa.). “That will require a campaign of surgical precision.”

Scott Clement contributed to this report.