Bernie Sanders recently leveled a charge against Hillary Clinton:

You can be a moderate. You can be a progressive. But you cannot be a moderate and a progressive. — Bernie Sanders (@BernieSanders) February 3, 2016

He repeated the charge in Thursday night's debate, and it became the pivotal point of conflict throughout the night.

The charge is false. But it's good that Sanders has made it, because it helps expose a key source of misunderstanding between the Clinton and Sanders camps. It is, as my philosophy professors used to say, an "illuminating error."

The two lenses through which we assess politics

There are two distinct sets of criteria by which we can judge any political candidate.

The first is ideological.

The easiest way to think about it is: If the candidate were emperor of the universe, allowed to shape the US to his/her liking, how close would that ideal world match our ideal world? Is it built around fairness? Diversity? Free markets? Christianity? Is there universal health care? An aggressive military? A robust social safety net?

Ideological assessment takes into account a mix of values and policies, but in the end it's about whether the candidate wants what we want for America. It's about, in George H.W. Bush's memorable phrase, "the vision thing."

The second is practical.

Given that ideal world, what's the best way to make progress toward it?

This is about, among other things, the candidates' theories of change, whether explicit or implicit. It's about how they pursue their plans, the compromises they make along the way, and, above all, the results they achieve (or at least, can be reasonably expected to achieve).

The two questions: What does the candidate want, and how effective will the candidate be at getting what he or she wants?

Tons of political arguments confuse these two types of assessment

A great deal of miscommunication in politics comes from mixing up the two sorts of assessments. It seems to me that's what Sanders is doing when he contends that you can't be a moderate and a progressive.

That's just wrong. "Progressive" is an ideological term. It refers to a position on an ideological spectrum, namely to the left. A progressive's opposite is a conservative.

"Moderate" is a practical term. It belongs to the second category of assessment. Broadly speaking, it refers to a candidate who focuses on consensus building and incremental progress, someone who doesn't believe the US political system is capable of sudden, lurching change, or just doesn't want that kind of change.

A moderate's opposite is a radical, someone who believes rapid, revolutionary change is both possible and necessary.

You can't be a centrist progressive, because "center" refers to that ideological spectrum. But you can certainly be a moderate progressive — someone who supports the ultimate goals of the left, but who believes a cautious and incremental approach is the best (or only) way to get there.

The distinction matters, because it helps map out the terrain each candidate want to fight on. In a nutshell, Sanders wants the contest to be about ideology, and Clinton wants it to be about practicality. He is the champion of ideological progressivism; she is the champion of practical moderation.

So when Sanders attacks Clinton over her progressivism, he is trying to pull the fight into ideology. Clinton defenders try to pull it back to practical matters, saying, no, it's not that Clinton doesn't share these big ideological goals, it's just that she realizes the only way to get there is through modest steps built upon existing programs. Pushing too much change too fast is dangerous (one of many lessons Clinton took from her 1993 health care debacle).

That was the dynamic behind the recent health care contretemps, in which Clinton argued for incremental progress based on Obamacare and Sanders partisans interpreted her as being against universal health care. It was behind the Wall Street contretemps, in which Clinton argued for incremental progress based on Dodd-Frank and Sanders partisans interpreted her as being against reining in Wall Street.

Similarly, Sanders proponents are quick to shift the discussion away from practical questions. All that's been said in support of Sanders's ambitious legislative plans is that there will be a "political revolution," which presumably involves either historic turnout, a historic shift of working-class white Republicans into the Democratic camp, or both. Few supporting details have been offered.

It all came to a head in Thursday night's debate, which finally brought the question to the surface. Sanders challenged Clinton ideologically, questioning her progressive bona fides, and Clinton challenged Sanders practically, questioning his "unachievable revolution."

On banks, on health care, on the death penalty, over and over the same difference was re-inscribed. It was a meaningful debate (the first of the cycle, really).

Ships passing in the night ... and opening fire on one another

Would that the online debates inspired by the candidates were as substantive and meaningful. Instead, the two camps seem to be moving closer to open warfare, at least in the hothouse environs of social media.

Sanders fans interpret every compromise, every note of caution, every incrementalist policy in Clinton's long record as a rejection of progressive ideology. Her refusal to echo Sanders's grand promises is interpreted as a rejection of his goals. The idea that Clinton and her supporters might believe equally in the values of the left but differ on strategy is rejected out of hand. There is zero presumption of good faith.

Meanwhile, Clinton fans are constantly lecturing Sanders supporters that they are naive, that they should abandon their ideological goals before they get a real chance to fight for them. A Democratic Party establishment that has allowed a down-ballot catastrophe over the past decade is telling them them to quiet down and let it take care of everything. Every expression of ambition is interpreted as a lack of knowledge about How Politics Really Works; again, there is zero presumption of good faith.

This is frustrating on both sides. But what makes it extra frustrating for Clinton supporters — many of whom are less than fully enthusiastic to begin with — is that the battle is not entirely symmetrical. As Ezra Klein has written, it is very difficult to make an inspiring political case for practical moderation.

"We probably can't achieve anything big, but I'll block Republican attempts to roll back Obama's achievements and accomplish what I can through executive actions" is not something passionate Democrats, the ones paying attention to the primary, want to hear. Voters hate hearing about what a candidate can't do. They hate hearing about process or constraints.

Battle-hardened pragmatism, which is Clinton's calling card, is not always easy to get excited about, even for those who believe it is well-suited to the political moment.

As Jamelle Bouie writes in a smart column this week, Sanders carved out a unique space in Vermont where he could be safely socialist for decades. (Where he needed to compromise, as on guns, he did.) Clinton has been in the muck, under heavy fire, from the day she left Arkansas. There has been no safe space for her.

She has made many compromises, beginning way back with prioritizing Bill's political career. As Rebecca Traister has articulated so well, in this column and others, many of Clinton's compromises are characteristic of the difficulty women of Clinton's generation had reaching the halls of power (and staying there). Entering public life as a wife was only the beginning.

Women, especially women around Clinton's age, see their own compromises and struggles in her. They know she has screwed up. They also know her ambition has been held against her in inescapably gender-inflected ways. They admire her, because she takes unending shit and just keeps going.

They think that bodes well for her resilience on the campaign trail and her practical wisdom in office. But that's not always an easy argument to make, especially in the face of an online legion of excited ideological partisans eager to hang her every sin on her every supporter.

I hippie punch more in sadness than in anger

As a dirty hippie of long standing, my heart has always been with those who itch for revolution. My ideal America is filled with solar panels, confiscatory tax rates on the rich, and quinoa-based lunches.

I have argued before in favor of activism, even aggressive, obstinate activism; on climate change alone, something like a revolution seems necessary. I have waged war on Hippie Punchers and Very Serious People — the loathers of the left and dispensers of stale Washington wisdom — my whole career.

But over time, I have grown extremely skeptical, not to say cynical, about the capacity of US political institutions to deliver dramatic change. And I've witnessed the cycle of hyperbolic liberal hopes followed by melodramatic liberal despair too many times.

Nobody likes hearing that they should rein in their ambitions in the name of realism, especially when reality is creeping oligarchy and militarism. Ralph Nader supporters hated hearing electability arguments. Obama supporters hated hearing that his soaring rhetoric was just an inspiring gloss on center-left technocracy all along. Single-payer supporters hated hearing that the public option had to be sacrificed to Joe Lieberman's ego.

Carbon tax fans hated hearing that Henry Waxman had to give away a bunch of pollution permits to get the cap-and-trade bill through the House. Keynesian backseat drivers believe to this day that Obama's economic stimulus could have been bigger if he'd just tried harder.

A bigger stimulus would have been great. So would a better, or indeed any, national carbon pricing policy. But there's always some new messiah or perfect policy just off to the left of what's possible, followed by betrayal and despair.

It makes the left congenitally unable to celebrate, defend, or even acknowledge the enormous progress that has been made under Obama. No incremental advance, even insuring tens of millions of people, looks good in comparison to the counterfactual case of leftist revolution, though it never seems to arrive.

Of course that's just my Hippie Puncher spin on things. Sanders supporters see it differently. They see the Democratic Party failing its constituents again and again through pathological caution and fealty to monied interests. They think for Democrats, "moderation" is a cover story for corruption. And they see a political revolution in the offing.

If the revolution doesn't arrive, it's not that it was doomed to fail in a system of government deliberately constructed to resist rapid change, in a country riven by deep and sincere cultural and ideological disagreements. It's just that leaders sold out to big money. The revolution never fails, it is only failed.

Here's the thing, though: The presidency is exactly the wrong place in American politics to look for a revolution. The president is constrained by layer after layer of checks and balances, veto points, entrenched interests, and institutional inertia. For a president in polarized times, progress comes not with a bludgeon but with a chisel.

The reason the left's revolution hasn't arrived isn't just that money has corrupted Washington, though it undoubtedly has. It's that half the country views massive new taxes and government spending programs with horror. And they are better organized, with possession at least one house of Congress and a large majority of statehouses. Resistance is not futile, but it is painstaking.