In the ad, Sanders says: “There are those who say we cannot defeat a corrupt political system and fix a rigged economy. But I believe we need to lift our vision above the obstacles in place, and look to the American horizon.”

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Sanders then envisions this future America as a place with affordable college, single-payer health care and a living wage for “all” (he has proposed a $15-per-hour minimum wage); and a place where retirement is a “time for rest and grandchildren” (he would expand Social Security). It’s a great ad: It suggests that if we summon sufficient political will, our political system and economy can be un-rigged and made to work for all of us.

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Sanders tends to paint our problems in dire, even apocalyptic terms — a massive, decades-long upwards redistribution of wealth has created an oligarchy that has captured our government, throwing the American Dream into grave doubt for people at all stages of life. But despite this, Sanders is optimistic. He hopes to inspire massive voter mobilization that can take Congress — and the country’s political economy — back from the oligarchy. He wants us to keep our eyes on the “American horizon” — the possibility of another great political transformation, of the sort that (our history tells us) really can happen. As Sanders puts it: “We need a political revolution where millions of people stand up and say you know what, that great government of ours belongs to all of us, not just the few.”

What would this revolution have to accomplish? At a minimum, it would likely have to win back the House of Representatives, since Congressional approval would be necessary for most of what Sanders wants to do, and it’s unlikely that Republicans would help him do it. Even Sanders himself has said he would not be so naive as to expect them to compromise with him.

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To determine what this might look like, I talked to David Wasserman, who closely tracks the ideological makeup of House districts for the Cook Political Report. Wasserman notes that Sanders’s main problem is that the median ideological makeup of all House districts is well to the right of center. Cook Political employs something called a Partisan Voter Index, or PVI, which essentially compares the presidential voting in each district to the national presidential voting, to situate each district ideologically against the country overall.

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According to Wasserman, the median PVI for all House districts in the country is +3 Republican — three percentage points to the right of the country overall. This is in keeping with broader population trends (and redistricting) that have redistributed Democratic voters in a less efficient way, resulting in a deeply polarized House map, meaning a lot of Republican districts that are very deep red, and a decline of swing districts.

To win back the House, given the current distribution of seats, Wasserman calculates that even if Dems hold all their seats and win a number of true swing districts, they’d still need to pick up as many as another two-dozen GOP held seats with a PVI of up to +3 Republican. As Wasserman sees it, there simply might not be enough voters sympathetic to the Dem agenda to flip all these seats in the near term.

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“For Democrats to win the House by even a single seat, they’d have to win all Democratic leaning districts, and all districts up to +3 Republican,” Wasserman tells me. “There aren’t enough disengaged young voters who are Bernie fans to make up the Democratic deficits in these districts. For Bernie to usher in a revolution that would reclaim some of these districts, you’d need to assume there are a lot of hard-core Bernie supporters who were disaffected in 2008 and 2012.” As it is, Cook Political Report currently sees only a dozen GOP-held seats as being true toss ups.

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None of this is to fault Sanders for talking generally about these matters. It is a huge positive for Sanders to be talking in blunt, energetic terms about the need to break big money’s influence over our politics and the need for massive voter re-engagement in the process. Sanders’s ability to highlight big problems with such bold strokes — energizing young voters — is something Hillary Clinton should learn from. There is a certain big-picture realism to Sanders’s insistence that an unabashedly ambitious, long-term, reform-oriented approach — rather than endless compromising and trench warfare — is needed to address the country’s deeper challenges.

But Sanders should be pressed for more detail about how his revolution would work. Does he really think the House can be recaptured during the first term of the next Dem president? If so, how would the numbers work? Or would that have to wait until the second term? Does Sanders believe outside pressure can prevail on moderate GOP members of Congress even as the GOP holds the House for the near term? If so, how? Sanders talks about reengaging disaffected voters and energizing independents behind his vision, but what would he do about the tendency of Dem-aligned voter groups not to show up in midterm elections? What is Sanders’s granular view of how the next few cycles will unfold when it comes to winning back ground on the level of the states, and what role does that — and the resulting redistricting of House seats in the next decade — play in this revolution?

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Hillary Clinton should be challenged on all this, too. She tends to acknowledge that the next Democratic president will be seriously constrained by the country’s deep ideological divisions, a GOP majority in thrall to the right, and the system’s built-in impediments to change. So how would she accomplish the things she promises to accomplish?