A specter is haunting Europe - the specter of nationalism.

For now, though, it's mostly just that: a specter. It hasn't been able to move beyond its phantasmological form and actually take power outside of Hungary and Poland. Everywhere else on the continent, mainstream parties have managed to keep the new, old breed of rabidly right-wing nationalists from taking the reins of government the past decade. How have they done this, even in the aftermath of the worst financial crisis in 80 years?

Well, it turns out, the center actually can hold - so long as the center-right helps out.

That, at least, is the lesson of the relentlessly technocratic Emmanuel Macron's bigger-than-expected blowout of the anti-immigrant and anti-European Union Marine Le Pen in France's presidential election. Even double-digit unemployment and semi-regular terror attacks weren't enough for Le Pen's far-right National Front to get anywhere close to 51 percent of the vote, especially as every other party treated them like political pariahs. Which is worth thinking about. Why did they?

After all, Republican elites in the United States ended up embracing then-Candidate Trump's equally extreme brand of populism that, like its French counterpart, called for banning Muslims, ripping up trade deals that were supposedly ripping the country off, and cozying up to Russia. So why didn't France's Republican elites - that's what they call their center-right party as well - do the same?

A few reasons. First off, Le Pen didn't take over her country's chief conservative party like Trump did. Instead, she took over her father's radical right party that's been trying to replace its Holocaust denial with a more socially acceptable hatred of Muslim immigrants. So there wasn't any partisan pressure on run-of-the-mill conservatives to support her. Second, in France, the ideological stakes between the center-right and center-left weren't so big that one felt like it couldn't vote for the other. The straitjacket that is the euro means that there's a lot of truth to the idea that they're just competing to see who gets to implement Germany's policy preferences. But third, and most important, is France's historical memory of itself. Not of 1789's "liberté, égalité, fraternité," but of 1940's "travail, famille, patrie" - the slogan of the collaborationist Vichy regime. That was the last time the far-right got control of the state, and it's the last time the rest of the country wants that to happen. The result has been a long-lasting taboo against being a stooge for a party that started out defending Nazi stooges.

The far-right, then, can only do as well as the center-right will let it. And that's not just true today. It always has been.

See, on the one hand, economists have found that it's the radical right that does the best after financial crises. When people aren't getting what they think they deserve, they look around to see who's getting more than they supposedly should - and then blame them. Sometimes that's the bankers who might have caused the crash, but more often it's people they come across in their everyday lives who they suspect of freeloading. Like the neighbor they don't think should be on disability, or the immigrant they're sure isn't paying taxes.

On the other hand, though, even this hasn't been enough for the far-right to ever get an outright majority - at least not on their own. That's the case whether you're talking about the National Front right now or the National Socialists (Nazis) in the 1930s. Remember, Hitler actually lost his bid for the German presidency in 1932, and the Nazis never won more than 37 percent of the vote in free and fair Reichstag elections. It was only when the conservative Paul von Hindenburg acquiesced to pressure to appoint Hitler to the Chancellorship that the Nazis were able to take over. So if right-wing extremists want to win power - assuming they're using ballots and not bullets - then they need right-wing non-extremists to go along with them.

They haven't in the French elections, or in the Dutch ones, and won't in the German ones later this year. But as Harvard's Daniel Ziblatt explains in "Conservative Parties and the Birth of Democracy," sometimes they're so weak that they can't help it, or feel like they can't afford not to. In other words, a center-right party that doesn't have the institutional wherewithal to control its base or the Machiavellian flair for reaching out to new constituencies might get hijacked by far-right activists or despair that using them is the only way it can win elections anymore. But, like most Faustian bargains, this doesn't tend to work out like they expected. Whatever tax cuts or tariff increases these vested interests thought they'd get are usually forgotten once the radical right actually gets its hands on the levers of power. And before you can say "Weimar," so are things like a free press and an independent judiciary.

The good news is that Europe doesn't have to worry about this spreading for now. But the bad news is that Europe knows it doesn't have to worry about this spreading for now. German Chancellor Angela Merkel has already said that they won't let French President Macron ease up on austerity, except maybe, possibly after he makes it easier for companies to fire workers (and she wins re-election herself). But even then, it's hard to imagine that she'd offer anything macroeconomically significant enough to make much of a difference. Europe seems to be betting that mainstream parties will just continue to quarantine the far-right, and be content to take turns crucifying themselves on a cross of euros, all to keep the 60-year dream of an ever closer union alive, and their people's savings from being devalued.

While you should never underestimate the power of the status quo, this is one of those things that works until it doesn't. Every year, there are more young people who, if they're lucky, might be able to find a job but not a career. And every year, there are fewer old people who grew up in the shadow of Vichy. Indeed, it wasn't idealistic twentysomethings who gave Macron and his doggedly upbeat message his biggest margins, but rather retirees who recoiled at the prospect of the far-right regaining power even 70 years later. That's why Le Pen père was only able to garner 18 percent of the vote when he made the second round of presidential voting back in 2002, but Le Pen fille was almost able to double that to 34 percent. Pressure is building. It won't burst today or tomorrow or even the day after that, but what about after the next recession? The far-right might not need much help by that point.

Which brings us to the real problem: everything seems impossible in Europe. It doesn't seem conceivable that a country could actually leave the euro, since just talking about that scares people into wanting to stay. Nor does it seem conceivable that they could build the United States of Europe it would take for the euro to work, since just talking about that scares people about losing their sovereignty. So they just muddle on. All the while, though, the far-right is slowly rising, and the center-right is slowly forgetting.

The nationalists of the world won't unite, but they might lose their metaphorical chains.