Hillary Clinton and Bernie Sanders and the other candidates took on gun control, Benghazi and other big issues at the first Democratic presidential debate of the 2016 race. Here are the highlights. (Sarah Parnass/The Washington Post)

In last night’s Democratic debate, there was only one question, to Bernie Sanders, on what may be the most difficult challenge that will confront the next president if he or she is a Democrat: What are you going to do about Congress?

We’ll get to the answer Sanders gave in a moment, but first, some context. When Barack Obama was elected, congressional Republicans made what was in some ways a strategically shrewd decision, that they were going to oppose him on basically everything. Because he started with huge majorities in both houses of Congress, he had an extraordinary record of legislative achievement in his first two years, that opposition notwithstanding. But in 2010 Republicans won the House, and four years after that they took the Senate. For all intents and purposes, legislating was over.

In those two wave elections of 2010 and 2014, a generation of extremely conservative Republicans who viewed all compromise as betrayal were elected, moving the party to the right ideologically and making it far more obstructionist. Now let’s say a Democrat wins in 2016. What happens then?

It’s almost a certainty that Republicans will retain control of the House. Democrats have a chance to win back the Senate (Republicans have to defend many more seats, because everyone who won in 2010 is up for reelection), but even if they do, it certainly won’t be with a filibuster-proof majority. Not only that, if Democrats make gains, it will be in those few competitive states and House districts, which would mean that the remaining Republicans would as a group be even more conservative than they are now. Are they going to be in the mood to work with a Democratic president?

So here’s what Bernie Sanders said when he was asked about this problem:

“Now, in my view, the only way we can take on the right wing Republicans who are, by the way, I hope will not continue to control the Senate and the House when one of us elected President. But the only way we can get things done is by having millions of people coming together. If we want free tuition at public colleges and universities, millions of young people are going to have to demand it, and give the Republicans an offer they can’t refuse.

“If we want to raise the minimum wage to $15 bucks an hour, workers are going to have to come together and look the Republicans in the eye, and say, “We know what’s going on. You vote against us, you are out of your job.”

In 2007, Mark Schmitt called the argument among Hillary Clinton, Barack Obama, and John Edwards the “theory of change primary.” As Clinton would describe it in speeches, Edwards thought you demand change, Obama thought you hope for change, and she thought you work for change. Sanders’ theory, as he lays it out here, is essentially that you force change, by making it too politically dangerous for Republicans to resist.

Which is realistic in one way and unrealistic in another. On one hand, Sanders is not bothering to indulge the dream that you can reach across the aisle and bring Democrats and Republicans together. In fact, no candidate from either party is saying that — and after the last seven years, who could do so with a straight face? But that’s a dramatic change from the last couple of decades.

Though they all ended up inspiring partisan passions, our last three presidents all ran as conciliators who could unite Washington and the country. Bill Clinton was going to create a liberal/conservative synthesis, a “Third Way” that could attract support from both parties. George W. Bush touted his record working with Democrats in Texas. “I want to change the tone of Washington to one of civility and respect,” he said in his 2000 convention speech.  Barack Obama, who became a national figure in a 2004 convention speech where he said, “there’s not a liberal America and a conservative America; there’s the United States of America,” thought that he could sit down with everyone, earnestly listen to their concerns, and bring them around at least some of the time. All three presidents failed at this goal.

But if Sanders is being realistic about the present, his portrait of his future presidency has a big problem, particularly in the House. Let’s say he succeeds in creating a mass movement behind parts of his agenda. Is he really going to be able to raise the political risk of opposing something like free public college tuition high enough to overcome House Republicans’ personal inclinations and their constituents’ wishes?

Imagine you’re a Republican representative who hails from a conservative district in Alabama or Idaho or Tennessee; we’ll call him Jim. Jim is right now stopping comprehensive immigration reform, which the GOP as a whole knows it needs to pass in order to have any chance of appealing to the growing Hispanic population. But Jim won’t sign on, because though that might be good for the party, it’s bad for him. His conservative constituents don’t want it, he personally doesn’t want it, and the only political risk he fears is a primary challenge from the right.

Is Jim really going to be scared and/or persuaded when a bunch of young people in America’s cities — even if there are millions of them — create a movement behind President Sanders’ plan for free college tuition? Don’t bet on it.

It should be noted that their obstructionism, and the demands it creates among their own constituents, may keep the GOP from winning the White House as long as it continues. But that’s not really a problem for Jim. Indeed, if they lose again, Jim and others like him will tell themselves that it was only because their nominee wasn’t conservative enough.

I’m talking about Sanders here because he’s the one who got that question last night, but I haven’t heard Clinton address this problem in a real way, either. And maybe there’s no good solution. I’m not sure how I’d tell them to answer it if I were advising them, at least not if they want to maintain the lofty, hopeful tone presidential candidates tend to use, where they present themselves as potent agents of change and renewal who can overcome any obstacle. No candidate is going to tell voters, “Here are the things I’d like to do, although, let’s be honest, I probably won’t be able to.” Even if it’s the truth.