When asked if each candidate “shares your values,” 42% of voters said Mrs. Clinton does, and 55% that she doesn’t. This might be problematic for the likely Democratic nominee, depending on who winds up as her Republican opponent. But, again, Mr. Trump’s numbers were worse. Only 35% said he shares their values, and 61% said he doesn’t.

All these numbers combine to make Mr. Trump the weakest Republican tested by Quinnipiac in head-to-head matchups against Mrs. Clinton, to whom he loses 41% to 47%. Among young voters, he loses by 20 points, 32% to 52%. He receives only 13% of Hispanic votes—less than half of what Mitt Romney did in 2012—to Mrs. Clinton’s 76%.

The Donald doesn’t compensate by beating Mr. Romney’s nearly 20-point margin among whites: He leads Mrs. Clinton among whites by only 12 points, 50% to 38%. So although Mr. Trump’s antics may not drive away his current supporters, they make him unlikely to win the White House.

Yet if the Republican field remains large and splintered through mid-March, Mr. Trump could become the Republican nominee by winning states with 25% to 30% of the vote. Then Democrats would attack Mr. Trump, a target-rich candidate, with an endless stream of ads.