But these forecasts are made based on simple models that do not include any information about the nominees (and implicitly assume, as Mr. Abramowitz notes, that the parties select mainstream candidates). In contrast, betting and prediction markets that take the likely party nominees into account put Mr. Trump’s chances of winning in November at approximately 25 percent.

Why, then, is Mr. Trump looking so likely to underperform in November? Even his unfavorability ratings may not fully explain his diminished chances. Candidates who are initially not seen as personally likable can overcome those perceptions, as Bill Clinton did in 1992, another “time for a change” election (though Mr. Trump’s ratings are worse, and he has been known for longer at the national level).

For instance, consider a counterfactual situation in which Ted Cruz, Mr. Trump’s chief rival, became the nominee. Mr. Cruz also had high unfavorable numbers before dropping out of the race, but if he had won the G.O.P. nomination under normal circumstances, most Republicans would have probably rallied around him (as they did around Mitt Romney in 2012), and the electorate would have eventually come to see him somewhat less negatively.

In Mr. Trump’s case, his reputation for irresponsibility and lack of political experience overshadow the “time for a change” message. He frequently calls for new leadership — a message that would normally be the strongest G.O.P. appeal in an election year when the economy is not weak enough to be the focus of the opposition’s campaign. But instead of being a referendum on the incumbent party’s failures, this election seems likely to turn on how voters view Mr. Trump himself. Many voters who might have been open to an alternative after eight years of President Obama will instead ponder whether it is too risky to vote for Mr. Trump, especially after Mrs. Clinton makes his qualifications the central message of her campaign.