Vermont Senator Bernie Sanders took time off from the campaign trail on April 15 to address the Pontifical Academy of Social Sciences, a kind of think tank located at the Vatican.

How to describe the address? It will likely have little direct impact on this year's presidential cycle. From a purely practical, tactical vantage point, Senator Sanders might have better spent his time in Albany, or Buffalo, given the impending New York primary.

Still, it is possible that we shall look back on this speech a few years hence and see within it a turning point in American political thought. For what Senator Sanders has done is return to an old source of inspiration for progressive politics and that is the social teaching of the Catholic Church.

Bernie Sanders quite properly began his speech with a nod to Pope Leo XIII's encyclical of 1891, Rerum Novarum. That encyclical was issued in a moment of crisis for western economies. Unrestrained capitalism had wreaked untold damage on Europe and the United States. There was fear of revolution. The Pope spoke urgently of the need to find "some opportune remedy . . . for the misery and wretchedness pressing so unjustly on the majority of the working class." With the rise of laissez faire economics, Leo went on, traditional legal restraints on capital were removed and so "by degrees it has come to pass that working men have been surrendered, isolated and helpless, to the hard-heartedness of employers and the greed of unchecked competition."

No wonder Bernie Sanders looked to Rerum Novarum for inspiration. In many parts of the world, including the United States, we are witnessing a return to the conditions Pope Leo denounced at the end of the nineteenth century. Thus Senator Sanders, speaking at the Vatican, sounded a theme drawn straight from Rerum Novarum: "The widening gaps between the rich and the poor, the desperation of the marginalized, the power of corporations over politics, is not a phenomenon of the United States alone. The excesses of the global economy have caused even more damage in the developing countries."

But Senator Sanders also turned to more recent papal teaching. He noted properly that Pope John Paul II in Centesimus Annus, issued in 1991, called on governments to ensure wages sufficient not only for subsistence but sufficient to support a family and even ensure some savings. And Senator Sanders eloquently endorsed Pope Francis' description of the modern human condition: The globalization of indifference." Emulating the Pope, Sanders pledged "to fight the economic juggernaut" and to reestablish "boundaries" around "the market economy."

So what makes Sanders' speech special? There is a remarkable convergence between Sanders' criticism of the rapacity of today's global marketplace and Pope Francis'. This should not in itself be surprising.

Beyond this convergence, however, the speech is important because of the possibility that today's progressive movement might once again draw intellectual sustenance from Catholic social thought. From the 1930's to the 1980's, this was a matter of routine. And so I might recommend that progressives take a look at the Catholic Church's teaching on the role of the state in ensuring economic justice.

And again, we might begin with Pope Leo, who charged the state with important functions: it must ensure that workers have regular days off and that working conditions be regulated so as to prevent injury to workers. Labor unions should be encouraged and child labor deterred if not abolished altogether.

These were responsibilities that Pope Leo placed on the state, as the ultimate guarantor of the common good. The assurance of "peace" and "good order" were among the chief duties of the state, and Leo drew from these requirements the affirmative obligation that the state must guard against class oppression (para. 36).

St. John XXIII, in his encyclical Mater et Magistra, published in 1961, developed the ideas found in Leo's encyclical. The purpose of the state, this holy Pope wrote, is "the realization of the common good in the temporal order" (para. 20). The state must "do all in its power to promote the production of a sufficient supply of material goods." The state must "protect the rights of all its people and particularly of its weaker members." Taxation must be fair and just, John XXIII wrote. Education must be made available and accessible. The state must not take the place of individual initiative, St. John XXIII continued, but it must "augment . . . freedom." John XXIII well recognized that true freedom is not lived in opposition to the state, but rather with the state ensuring the prerequisites for real human flourishing.

John Paul II, some neo-conservative commentators notwithstanding, stood firmly within this tradition in his encyclical Centesimus Annus. John Paul II endorsed the "natural human right" to form associations, including labor unions. The state, John Paul insisted, must ensure the implementation of this basic associational right. The state must also make certain that just wages are paid, and see to the protection of "the defenseless and the poor."

I could go on, but I think the point is made. One of the most important contributions that Catholic social thought can make to today's progressive politics is a theory of the state as guarantor of a just and fair economic playing field. Bernie Sanders and others would be well-advised to draw deeply from this tradition.

In doing so, they would find themselves at odds with the last three-plus decades of political discourse, which has been all about de-legitimizing the state. When Ronald Reagan said in 1981 that "government is the problem, not the solution," he likely did not believe it himself. But his rhetoric was careless. And surely it stands behind much of the reckless talk and dangerous politics emanating from Donald Trump, Ted Cruz, the Tea Party fanatics, and the Ayn Rand libertarian right.

What progressives must do, in other words, is confront this toxic legacy and reinvigorate the place of the state in American politics. The state serves the common good. Only if we make that case, can we make the further case that the state should protect the panoply of economic rights that we have been talking about -- from the right to organize, to the right to a living wage, to the right to enjoy a just division of the goods and material resources of society. Catholic social thought, with its robust theory of the state and its strong conception of justice, provides a rich font of ideas and progressives would be do well to consult it.