To go from a protest candidate to one with a realistic chance of winning, Sanders will have to get more specific about his policy proposals. Photograph by KENA BETANCUR/AFP/Getty

During the past few days, a number of liberally inclined commentators have published pieces querying Bernie Sanders’s domestic program, particularly his “Medicare for all” health-care proposal. That’s not particularly surprising. With polls showing Sanders leading, or challenging Hillary Clinton for the lead, in Iowa and New Hampshire, his proposals demand inspection. Until recently, there hadn’t been very much of this.

The latest piece to catch my eye was written by Michael Linden, a former policy adviser on Capitol Hill, who has also worked at the Center for American Progress, a think tank founded by Clinton’s campaign manager, John Podesta. The article, which Linden published on Medium, is entitled “The four policy reasons why I support Hillary Clinton in the Democratic primary.”

Linden writes that Clinton’s domestic proposals, which include raising the minimum wage, eliminating tax loopholes enjoyed by the rich, expanding preschool, and trying to make college more affordable, “push the boundaries of the possible.” He argues that they are also rooted in evidence, confront unavoidable trade-offs, and aim to help the poorest and most vulnerable members of society. To those Democrats who say that Clinton’s program doesn’t go far enough, Linden replies, in essence, that it is important—and actually more progressive—to be realistic about what can be accomplished. He writes:

A boundary-ignoring approach is less likely to produce actual policy change than boundary-pushing. You can be frustrated with the pace of that change and you can wish that our system allowed a president to enact the agenda that got him or her elected, but the historical fact remains that the few cases when federal policy really did take an enormous leap forward are the exception, rather than the rule. So until there is a plausible case that we are on the precipice of one of those rare exception moments, I prefer the approach that offers the possibility—however small—of real gains.

Linden doesn’t question the attractiveness of the vision Sanders is promoting—an America with a universal public health-care system, universal paid leave, free college tuition at state colleges, and a huge infrastructure program. Ultimately, a single-payer health-care system “could offer us enormous benefits,” Linden writes. But he also points out that Sanders, in order to finance such a system, would have to raise taxes sharply. “There’s no realistic chance of getting even a decent fraction of what Sanders has asked for. And if that’s the case, then Sanders has clearly not done the hard work of figuring out priorities, of operating within constraints, of balancing trade-offs,” he writes.

The argument is forcefully made. However, Sanders, as I understand him, isn’t claiming that his ambitious and costly program is realistic in today’s Washington. To the contrary, he says that the political system is so broken, and so in hock to big money, that it is virtually impossible to effect nearly any substantive progressive change. The only way to make big changes, Sanders argues, is to create a mass movement that faces down corporate interests and their quislings. Once this movement materializes, all sorts of things that now seem out of the question—such as true universal health care, free college tuition, and a much more progressive tax system—will become possible.

This, surely, is what Sanders means by the term “political revolution,” which he uses all the time. In a piece published on Thursday, the Washington Post’s Greg Sargent highlighted two things that Sanders had told Andrew Prokop, a reporter from Vox. Speaking in Iowa last week, the Vermont senator said that real change only comes about “when people on the bottom begin to stand up and say enough is enough. That’s true of the civil-rights movement, it is true of the women’s movement, it’s true of the environmental movement, of the gay movement.” In an earlier interview with Prokop, Sanders had differentiated his approach from the President’s: “The major political, strategic difference I have with Obama is it’s too late to do anything inside the Beltway. You gotta take your case to the American people, mobilize them, and organize them at the grassroots level in a way that we have never done before,” he said.

All politicians talk about mobilizing the populace: Sanders has thus far done a better job of it than most. Building on the success of the Occupy Wall Street movement and the progressive networks that it spawned, he has attracted huge crowds to his events, signed up thousands of volunteers, and, at last count, attracted more than 2.5 million individual campaign contributions. That is very impressive, especially when you consider that Sanders was widely written off when he entered the contest last April.

Until recently, Sanders has focussed more on his instrumental goal—building up a movement to enlarge the political space—than on the actual policy measures he would introduce if his candidacy were successful. Indeed, he can appear a bit reluctant to engage in a discussion on policy details. Toward the end of Sunday’s Democratic debate, NBC’s Lester Holt asked the three candidates how they would implement their biggest ambitions in their first hundred days in office. Sanders was careful to place his policy goals in the context of his larger narrative: