One of the most revealing exchanges in Saturday's Democratic debate came when moderator John Dickerson pressed Bernie Sanders on what, exactly, he meant by that "political revolution" he always talks about. With such widespread GOP control of statehouses, Dickerson asked, wasn't there actually "a conservative revolution going on in America right now?"

So Sanders explained exactly what he meant. Here's what he said:

We are gonna do a political revolution which brings working people, young people, senior citizens, minorities together. Because every issue that I am talking about- — paid family and medical leave, breaking up the banks on Wall Street, asking the wealthy to pay their fair share of taxes, rebuilding our crumbling infrastructure, raising the minimum wage to 15 bucks an hour -- every one of those issues is supported by a significant majority of the American people. Problem is, that as a result of a corrupt campaign finance system, Congress is not listening to the American people. It's listening to the big money interests. What the political revolution is about is bringing people together to finally say enough is enough. This government belongs to us. Not just the billionaires.

This is really a key — maybe the key — question of Bernie Sanders's candidacy. Can he really assemble a national coalition that defies the apparent laws of national politics — turning out millions of people who usually stay home, and winning over many white voters and seniors who usually vote for Republicans?

If he can, he might be able to enact an agenda of sweeping change. But if he can't, a Bernie Sanders presidency might not look all that different from a Hillary Clinton one — and perhaps could never happen at all.

Because Sanders's political revolution is mainly about mobilization. He wants to mobilize not just traditionally liberal demographics but also people who don't usually vote, and Republican-leaning groups like senior citizens and working-class white people. He thinks he can do so by unifying them around an agenda that challenges the power of the wealthy. If these people actually a) turn out to vote, and b) keep actively pressuring their representatives to pass an agenda, Sanders argues, then true change can happen.

Why hasn't it happened already? The problem, he believes, is that Americans are not convinced the Democratic Party will fight for them. As Sanders told Vox:

Are the Democrats 10 times, 100 times, better on all of the issues than the Republicans? They surely are, but I think it would be hard to imagine if you walked out of here or walked down the street or went a few miles away from here and you stopped somebody on the street and you said, "Do you think that the Democratic Party is the party of the American working class?" People would look at you and say, "What are you talking about?" There was a time — I think under Roosevelt, maybe even under Truman — where it was perceived that working people were part of the Democratic Party. I think for a variety of reasons, a lot having to do with money and politics, that is no longer the case. In my view that is exactly what shouldn't be happening. Instead of spending all of our time raising money, I think we should go out organizing people and getting them to unite around a progressive agenda which expands the middle class, which tells the billionaire class that they cannot have it all, which says to corporate America, "You're going to have to start paying your fair share of taxes," which says we're going to raise the minimum wage, we're going to make college available to all regardless of their income, that we are going to have pay equity for women workers, that we are going to create millions of jobs rebuilding our crumbling infrastructure. You need a progressive agenda, then you need the ability to go out and organize people. When that happens, things change here; it's not the other way around.

Sanders argues that he's managed to do just that in Vermont. It seemed inconceivable that an independent "democratic socialist" could win a House or Senate seat, but he did both by winning over white, rural, and working-class voters. Indeed, he was the first independent elected to the US House of Representatives in 40 years. This past success is why he's so convinced that his model can work nationally, too.

But if Sanders can't manage to conjure up this unprecedented mobilization, he himself has admitted that his presidency would fail to bring about major change. "Sixty percent of the American people are not likely to vote in the coming election," he said in Waterloo, Iowa, shortly before the 2014 midterms. "You think you can bring around change with that dynamic? You can have the best human being in the world in the White House fighting all the right fights, and he or she will fail."

So in a sense, Sanders's campaign could be either self-proving or self-refuting. If his theory of change is right, his unusual brand of politics will change everything — massive support from a newly mobilized public would prove all the pundits and the naysayers wrong and help a democratic socialist do the seemingly impossible and actually win the presidency in the United States of America. And a world where Sanders can win is a world where Sanders's agenda may well be able to pass.

But if he's wrong — if he manages to win the primaries anyway but the foot soldiers he hopes will join his political revolution never show up — his nomination could end up a fiasco for the Democratic Party.