Kennedy also campaigned on the dignity of work over welfare. In a TV commercial, he declared, “I think welfare is demeaning and destructive of the human being and of his family.” He didn’t blame “welfare queens” for cheating the system, as Ronald Reagan later would, but said he envisioned a policy of full employment in which a person could say to himself: “I helped build this country. I am a participant in its greatest public venture.”

On issues of national security, Kennedy took a principled position in opposition to the Vietnam War — whose very morality he questioned — but threaded the needle in a way that also made clear to working-class voters that he differed sharply from upper-middle-class white college students who avoided service or even sympathized with the North Vietnamese Communists. At Notre Dame, Kennedy was booed for saying college draft deferments should be abolished. “You’re getting the unfair advantage while poor people are being drafted,” he said. Remarkably, in Indiana he polled as well among those who favored Johnson’s conduct of the Vietnam War as he did among those who opposed it.

Kennedy’s campaign to woo working-class voters across racial lines worked. The candidate most identified with advancing civil rights did well not only with black and Hispanic voters but also among working-class whites, some of whom had supported Wallace’s segregationist candidacy in 1964.

A half-century later, how could progressives try to rebuild the Bobby Kennedy coalition? Kennedy’s appeal was based in part on being the brother of a revered and martyred president, of course, and the most salient issues were different in 1968 than they are today. But Kennedy stressed fundamental themes that travel across time and transcend specific policy issues.

First, to appeal to a sizable number of white working-class voters in 1968, Kennedy did not forfeit his basic principles or change his positions on civil rights, or war and peace — and neither should progressives today. Ignoring the rights of women, gay people and people of color is both morally wrong and politically stupid if your aspiration is an inclusive populism.

Second, progressives should fight for economic justice in a manner that is relentless rather than episodic. On the campaign trail, Kennedy consistently hit themes of economic inequality and named the names of wealthy individuals, like the oil tycoon H. L. Hunt, who paid little in taxes. By contrast, in the final weeks leading up to the 2016 election, Hillary Clinton de-emphasized economic issues in favor of attacks on Mr. Trump’s qualifications, according to research by Democracy Corps and the Roosevelt Institute, and his support among white non-college voters rose considerably. Progressives also need to vigorously punish Wall Street malfeasance. It is difficult to imagine that Kennedy, a tough prosecutor, would have argued, as some members of the Obama administration did, that some companies are “too big to jail.”