DOVER, New Hampshire — Former Vice President Joe Biden took multiple swipes at his Democratic rivals, urging them to resist the temptation of turning on one another as the crowded primary field jostles for the right to challenge President Trump next year.

“The good news about being a front-runner is that you’re front-runner. The bad news about being front-runner: Everyone is behind you, looking,” Biden said Friday at a campaign stop in Dover, New Hampshire. “The last thing we need, to use Barack’s phrase, is form a circular firing squad here. We should be arguing about the future,” he added, referring to the warning former President Barack Obama issued in April.