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Watching the Brexit debacle from afar, it seemed impossible to understand how the Labour Party could know full well it needed to win a national election in order to prevent an irreversible setback, yet harness itself to Jeremy Corbyn, whose toxic leadership made victory nearly impossible. The rise of Bernie Sanders, at a moment when Donald Trump is accelerating his war on the rule of law, is retroactively illuminating. A liberal party drifting helplessly along as a small radical cabal steers it toward likely catastrophe? I didn’t think it could happen here.

At the heart of Sanders’s campaign is a hard-core socialist vanguard which is indifferent to the Democratic Party except as a potential vessel for the Bernie revolution. Their calculation is perfectly rational. Even if Sanders is likely to lose, the small chance of success is worth the risk to a party they don’t care for to begin with. What is odd is watching rationalizations take hold among a much larger group of progressives who very much do care about denying Trump a second term, and who have explained away the risks of a Sanders nomination with a series of fallacies.

The first of those is a confusion over what it means to predict an outcome. “The truth is we are all clueless about what voters want or will accept,” argues conventional-wisdom-monger Jim VandeHei, in a signal of how deeply the anti-probabilistic fallacy has spread. It is true that there is uncertainty attached to every outcome. The talking heads who guarantee Sanders will lose are wrong — any nominee might win, and in a polarized electorate, both parties have a floor of support that gives even the most toxic candidate a fighting chance. In 2016, Trump was the most unpopular candidate in the history of polling, but he squeaked into office because everything broke just right for him. It could happen for Bernie, too.

But to concede that we cannot be certain about the future does not mean we know nothing. An imperfect comparison might be to predicting the outcome of sporting events. You don’t know the outcome in advance, but it is usually possible to make probabilistic predictions. Those predictions are wrong all the time. But it would be silly to conclude that, just because upsets happen, every game should be treated as a coin flip. A huge amount of pro-Sanders commentary is based on simplistically conflating the correct claim that we lack perfect clarity with the incorrect claim that we have no clarity at all.

A close relative to the notion that outcomes are completely unknowable is the notion that the entire electability calculation is a kind of hoary pseudoscience. After all, if we have no way of predicting what voters will want in November, why compromise in advance in pursuit of a mythical swing voter whose preferences are mysterious?

The grain of truth in this argument is that, historically, it is difficult to predict candidate performance in advance. The gigantic flaw in the logic, however, is that the backlash against electability has a behavior-shaping component that undermines itself as soon as it is put into practice. Historically, major candidates try pretty hard to avoid taking extremely unpopular positions. If candidates stop following this principle, on the grounds that electability is a myth, then the risk they’re dismissing will grow. It’s a bit like emphasizing the fact that animal attacks at zoos are rare. If people decide this means they can start flinging themselves into lion dens, then the zoo-safety stats will go south pretty fast.

The Democratic primary can be seen as a series of candidates goading each other to jump into the lion’s den. All the candidates have exposed themselves by taking at least a few unpopular positions, but none have gone quite as far as Sanders. What makes Bernie’s profile uniquely toxic is the way his liabilities all reinforce each other. He combines discrete, deeply unpopular policy positions with an unpopular socialist label, which in turn reinforce the fact that his campaign is premised on radically changing the economy, the one thing most voters believe Trump has done well. His historic statements praising various leftist dictators reinforce the impression of kookery.

One of the things Democrats have been telling each other is that it doesn’t matter what attacks Trump uses, because he’s going to make hyperbolic charges against them no matter what. And yes, Trump would call any Democrat an open-borders socialist who will throw everybody off their private insurance and drastically change the economy. But accusations work better when the target agrees with them. In 2016, Trump drew a lot of blood calling his opponent corrupt, but if Hillary Clinton had openly promised to use her office as a platform for lawbreaking and self-enrichment, it’s fair to say she would have done worse.

All the Democratic candidates have shortcomings in their record or their personality that Trump could exploit. What sets Sanders apart from the field is that his weaknesses aren’t ugly comments from the past or unflattering episodes in his history, but his own core program. He has functionally reverse-engineered Trump’s preferred attacks into a series of campaign promises.

Just how damaging these positions will ultimately prove in the general election is impossible to measure. The effect is probably not zero. As a general rule, politicians for every position from dog catcher on up understand that advocating unpopular things makes winning elections harder. Not impossible, but harder. All things being equal, a candidate for dog catcher who promises to round up and cook stray pets at random has less chance of winning. Sanders can still overcome this and win, of course. One can even imagine circumstances — like a sudden, deep recession — in which he would probably win. One could likewise imagine a town that elects the dog-barbecue candidate for dog catcher. All that said, the scale of the downside risk seems unnervingly high.

But what about those polls showing Bernie doing about as well as anybody else against Trump? “Virtually every national and swing state poll shows Sanders tied with or beating President Trump,” notes VandeHei. Alas, as political scientist Brendan Nyhan has explained, trial-heat polling at this stage of the race has little to no predictive power. The likely reason for this surprising fact is that trial-heat polling during a primary is distorted by the primary itself. Candidates who are targeted by opposing party messaging will tend to sink, while for those who are spared, it will rise.

In 2008, Republicans began shifting their attacks to Barack Obama after he took a delegate lead over Hillary Clinton. By the end stages of the primary, Clinton was outperforming Obama in polls against John McCain. And indeed, Clinton’s popularity continued to rise for years after Obama became the face of his party, and Clinton was spared the brunt of Republican hostility. But this hardly proved Hillary Clinton would have made a stronger 2008 nominee than Obama. It simply displays the fleeting benefit of drafting off the front-runner’s position as the principal target of intraparty attacks. Notably, Trump has been directing his criticism at almost every candidate except Sanders, whom he, in fact, frequently defends as the innocent victim of a rigged process.

The Sanders campaign’s standard-issue response to electability concerns is to tout his ability to generate enthusiasm which, he predicts, will produce the highest voter turnout in history. But his campaign has failed to produce anything like this so far. Even given the near-optimal conditions of the campaign’s early stages, when his organizers could devote months concentrating all their resources on a handful of low-turnout contests, they have not yielded any measurable spike. Iowa and Nevada had lower turnout than the last contested primary. New Hampshire had somewhat higher turnout, though the increase was concentrated in areas won by Bernie’s opponents.

Indeed, since his 2016 campaign, the whole premise of the Sanders revolution has disintegrated. For a fleeting moment in 2016, it seemed plausible that broad dismay with capitalism had spread to a point where a majority of voters were ready to embrace radical change, from left or right. Instead, satisfaction with economic conditions has risen to the highest level in two decades. They are poised to seize the presidential nomination from a divided and disoriented party without coming up with any blueprint for general success more confidence-inspiring than “who knows, anything can happen.”

In Britain, the Corbyn faction wrested control of the Labour Party, insisting its populist message would pull the working class away from the Conservatives. The Corbynites managed to gain control of the party and hold it, even as the signs of doom spread everywhere. Indeed, many of the same cadres who spent years insisting Corbyn would somehow prevail, dismissing all skeptics as paid-off shills for a discredited neoliberal regime, have simply shifted to making the exact same claims now on behalf of Sanders.

For Bernie’s most fanatical adherents, none of this matters. Sanders is a completely unique asset for the left — a proud socialist who holds national office — who will be too old to run again by the next election (if he isn’t too old already), and who has no short-term successor. They have every incentive to imagine away his many flaws and seize on the real but unlikely possibility he can defy the odds and win. What is shocking is observing how many other progressives have joined in this fantasy with them.