In 2008, Democratic nomination contenders Barack Obama and Hillary Clinton tore each other to shreds. Some of it was political and substantive while some of it was personal and slimy. But none of it would be considered “civil” by the standards now being applied to today’s Democratic primary.

It’s not a coincidence that the right flank of the party started finger-wagging about divisiveness, incivility, and disunity precisely when the left flank began to seriously threaten its dominance. As the left wing gains momentum, the party establishment’s tolerance for legitimate criticism wears thinner, and the range of topics considered off-limits or below-the-belt expands. New behavioral norms have appeared out of thin air: suddenly political criticisms of opponents, no matter how legitimate by traditional standards, are a bridge too far, and are even alleged to help Donald Trump. At least, if the criticisms are flying from left to right.

It should be obvious that all the scolding about divisiveness is merely a reflexive defense mechanism, an easier task for centrists than defending their politics on its merits. It deserves little more consideration than that. But if we do decide to subject to closer scrutiny the idea that criticism in a primary imperils the winner’s prospects in the general election, we find that it quickly falls apart.

To make informed decisions about who to nominate for the general election, the electorate should be able to get a good look at the candidates, warts and all. While there’s little value in personal smears and ugly innuendo, vetting candidates’ records and histories is indispensable. General elections are gloves-off affairs: vicious attacks are guaranteed. You don’t want to end up in a situation where you’re asking millions of people to defend a candidate against a charge that everyone knows deep down is indefensible. Therefore, it’s best to air dirty laundry during the primary and let voters decide what they can tolerate in a general election.

That’s why Bernie Sanders had nothing to apologize for this week when his senior campaign adviser David Sirota sent an email to campaign supporters promoting an op-ed written by campaign surrogate Zephyr Teachout. The op-ed was titled, “‘Middle Class’ Joe Biden has a corruption problem — it makes him a weak candidate.”

Law professor Teachout has made her name running for office against self-interested politicians, and is the author of the book Corruption in America. This is her wheelhouse. Biden, she wrote, has

perfected the art of taking big contributions, then representing his corporate donors at the cost of middle- and working-class Americans. Converting campaign contributions into legislative favors and policy positions isn’t being “moderate”. It is the kind of transactional politics Americans have come to loathe.

Biden and his campaign took umbrage at Sirota’s promotion of the op-ed and promptly launched into the unity-and-civility routine. Various liberal pundits piled on, calling it “a bad look” for Sanders. To the dismay of many of his supporters, Sanders apologized to Biden. It turned out the apology may have been a bait and switch — later that evening, the Sanders campaign published an ad eviscerating Biden’s record on Social Security.

But politicking aside, the truth remains that Biden’s political history is checkered with ethical grey areas, even if many pertain to actions that are perfectly legal, and that this will be a massive liability for him in the general election. The points Teachout raised in her op-ed should give everybody who wants to beat Trump pause about the viability of a Biden campaign.

You see, Democratic candidates might be able to browbeat each other into retracting criticisms by histrionically taking offense, but that won’t work on Trump. In fact, if he’s smart, he’s watching right now to see what makes candidates the most squeamish, what they’re most sensitive about, what they’re least equipped to explain. Whatever topic makes a candidate the most uncomfortable is what Trump will likely zero in on in the general election. It’s not airing criticism that gives future opponents ideas for lines of attack; it’s panicking, evading, or otherwise failing to coolly respond to them.

If Biden were to run against Trump, his perceived corruption would surely get top billing. As Jacobin’s Branko Marcetic has written:

The Biden family’s propensity for engaging in money-making ventures that — gee whiz, just somehow seem to constantly overlap with Biden’s political career — will make him a perfect foil to Trump. Whether it’s Biden’s son, Hunter, being hired as a lobbyist for a Delaware credit card company whose favored legislation Biden was voting for; Biden’s brother mysteriously getting hired by a mid-size construction firm shortly before it received a $1.5 billion government contract; or Hunter, again, joining the board of a corruption-tainted Ukrainian gas producer while Biden spearheaded US policy on Ukraine.

We know Trump has been planning to attack Biden on this issue for ages — in fact, that acute interest in the subject is what started the whole chain of events culminating in the current impeachment proceedings. And once impeachment fails and Trump is acquitted, it will be easier than ever for him to press the case that Biden is a shady character.

It doesn’t matter that Trump himself is egregiously corrupt. In fact, that’s precisely why it’s a terrible idea to send Biden up against him. Trump’s corruption needs to be central to the campaign against him, but Biden doesn’t have much of a leg to stand on. His son really did receive $600,000 a year from a Ukrainian gas company in exchange for doing nothing when Biden was vice president. He really does have a well-documented tendency to act on behalf of the same corporate interests that have bankrolled his political career — particularly finance, health care, and fossil fuels, as Teachout observes.

These unflattering truths will improve Trump’s chances if Biden’s the nominee. As Teachout wrote, nominating Biden “will allow Trump to muddy the water, to once again pretend he is the one ‘draining the swamp,’ running against Washington culture.” The fact that Trump is worse than Biden will not neutralize the threat. Biden’s ethical record will still dampen enthusiasm for him and increase the likelihood of defeat. This will pose a problem especially with working-class nonvoters, who are already likely to consider politicians uniformly untrustworthy. Without those voters, the path to victory over Trump narrows.

By contrast, Trump’s got nothing good on the other front-runner in the race. What will he say about Bernie: that he’s a crazy socialist? Bernie has always been consistent and honest about his political views, and his short-term program is overwhelmingly popular. He’ll simply respond, “So you think it’s crazy to make sure everyone has health care, housing, education, a living wage, a clean planet, and a secure retirement?”

Whatever Trump says next, at least now we’re having a useful and generative political conversation as a country — an argument that’s a long time coming and one we actually stand a chance at winning, given the intensity of people’s dissatisfaction with the status quo. Plus, it’s amusing to imagine Trump calling Bernie’s proposals unaffordable and unworkable, and losing all the Obama-to-Trump converts that he won by posturing as the change candidate in 2016.

Teachout’s op-ed was spot on. “We don’t have to choose Biden’s way, which would give Trump a perfect foil,” she writes. “The 2020 election should be about a crystal clear contrast between truth and lies, corruption and integrity, compassion and cruelty.” The candidate who allows us to draw the sharpest contrast, of course, is the one perceived as most honest: Bernie Sanders.