Jill Biden laid out the political calculus of her husband’s presidential campaign in extraordinarily blunt terms on Monday, directly acknowledging that some voters may prefer other candidates but urging them to support Joseph R. Biden Jr. anyway, in an effort to defeat President Trump.

As Mr. Biden, the early poll leader, works — and sometimes struggles — to excite a Democratic base that has moved left since he last ran for office, Dr. Biden, campaigning in New Hampshire, called on Democrats to prioritize perceived electability over enthusiasm for individual contenders or their policies.

“You may like another candidate better, but you have to look at who is going to win,” she said, addressing a gathering of educators. “And if education is your main issue, Joe is that person.”

“Your candidate might be better on, I don’t know, health care than Joe is, but you’ve got to look at who’s going to win this election,” Dr. Biden said. “And maybe you have to swallow a little bit and say, ‘O.K., I sort of personally like so-and-so better,’ but your bottom line has to be that we have to beat Trump.”