The usual question that gets asked about Bernie Sanders's political ideology is what does it mean that Sanders is a democratic socialist?

But the reverse question is, in some ways, more revealing: What does it mean that Bernie Sanders isn't a Democrat?

Sanders's ideas and voting record are pretty standard for a liberal Democrat. His big speech defining his ideology mostly quoted FDR — a Democrat — and laid out policies that put him squarely in the liberal mainstream. Indeed, according to Keith Poole and Howard Rosenthal's data, Sanders isn't even the Senate's most liberal Democrat right now — he trails Elizabeth Warren and Tammy Baldwin.

Moreover, the Vermont senator's life would, arguably, be easier if he were just a Democrat. He caucuses with Democrats in the Senate. He is running for the Democratic Party's presidential nomination. Democrats are one of this country's two major political parties, whereas there's no socialist movement in America to speak of.

So there's at least a bit of a mystery here. Sanders could call himself a Democrat — it would fit his ideology, and it would fit his ambitions. But he steadfastly refuses. Understanding why is, I think, crucial to understanding not just Sanders but his support.

Sanders doesn't believe Democrats are the party of the working class — at least not anymore

When I interviewed Sanders, one of the most interesting exchanges came when Sanders said that as president, he would pass the Employee Free Choice Act — legislation that makes it much easier to organize a union.

In 2008, then-candidate Barack Obama also promised to pass the Employee Free Choice Act, and pretty much every Democrat in Congress made the same pledge. But after organized labor helped Democrats win the presidency and huge congressional majorities, a massive lobbying effort by the business community killed EFCA. What would be different this time, I asked?

Sanders's answer, put simply, was grassroots support. But didn't Obama have a lot of grassroots support in 2008? That led to Sanders's real answer:

The Democrats, to a much-too-great degree, are separated from working families. 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.

Sanders's socialism is at least partly a signaling device: He's signaling that he's not a member of the modern Democratic Party, that he hasn't made the compromises with big money that other modern Democrats have made, and that he won't get into office and fall prey to the same advisers that other modern Democrats have picked.

Sanders's willingness to pick an ideological label that includes the word "socialist" acts as kind of proof of concept: Anyone who would spend years defending socialism in America isn't going to get elected and appoint a raft of investment bankers to their Cabinet.

I doubt Sanders or his supporters would much like this analogy, but there's an echo of the Tea Party here. Poll after poll showed that Tea Partiers didn't believe anything that was particularly unusual for conservative Republicans. But conservatives were fed up with the Republican Party — with its compromises, its failures, its corruption — and embracing the Tea Party label was a way of signaling a break not so much with Republican policies but with the cultural and political pressures that took firebrand conservatives and made them into Washington Republicans.

Sanders's campaign is premised on inspiring a "political revolution" led by the working class, and Sanders thinks the Democratic Party's brand is too toxic for a Democratic candidate to be the leader of that revolution. His main issue is money in politics — that, he argues, is the force that keeps the working class from being better represented in Washington — and when it comes to taking money from big donors and bending toward their will, he thinks the Democratic Party is nearly as bad as the Republican Party and voters know it. If Sanders is going to pick up those voters and engage them in a class-based political revolution, he can't do it as a Democrat.

Sanders's socialism, in other words, is a costly — and thus believable — signal that he isn't going to get elected and suddenly start doing fundraisers with Robert Rubin. That signal has powered a truly extraordinary primary campaign — and it suggests that there may be more liberal dissatisfaction with the Democratic Party than many in the party realize.