“We’re talking about somebody who has the passionate devotion of a minority and alternately scares, appalls, angers — or all of the above — a majority of the country,” said Henry Olsen, a conservative analyst. “This isn’t anything but a historic election defeat just waiting to happen.”

What could ensure a humiliating loss for Mr. Trump in November are his troubles with constituencies that have favored Republicans in recent elections. Among independents, a group that Mitt Romney carried even as he lost to President Obama in 2012, Mr. Trump would begin the fall campaign at a considerable disadvantage: 19 percent have a favorable opinion of him, but 57 percent view him unfavorably, the Times/CBS survey found. Given his loathed standing among Democrats and the possibility that many in his own party would spurn him, Mr. Trump would need to invert his numbers among independents to even be competitive in November.

With white women, a bloc Mr. Romney easily won even in defeat, Mr. Trump is nearly as unpopular: 23 percent view him favorably, while 54 percent have an unfavorable opinion of him. And that was before Mr. Trump attacked Senator Ted Cruz’s wife, ridiculed a female reporter against whom Mr. Trump’s campaign manager was charged with committing battery, and suggested that women who have abortions should face criminal punishment before reversing himself.

Mr. Trump’s penchant to offend and his household-name celebrity are a potentially lethal combination, as most voters have both firm and deeply negative opinions of him. His incendiary comments about minorities and the disabled, and proposals to bar Muslims from entering the United States or to force Mexico to pay for a wall on the southern border, have resounded so widely that half of all voters said they would be scared if he were elected president, according to the Times/CBS poll.

“There is no precedent for this,” said Neil Newhouse, a veteran Republican pollster. “In the modern polling era, since around World War II, there hasn’t been a more unpopular potential presidential nominee than Donald Trump.”