Former vice president Joe Biden claims that the nearly year-long assault on his record has been entirely unsuccessful, showing that he can take a pummeling and keep fighting. Former South Bend, Ind., mayor Pete Buttigieg has absorbed shots on his fund-raising practices, his record on race and his experience. Sen. Elizabeth Warren (D-Mass.) showed on Medicare-for-all that she had a glass jaw. Goaded into presenting her far-fetched funding scheme, she then retreated via a go-slow implementation plan. Sen. Amy Klobuchar (D-Minn.) was put through the ringer over her management style.

Yet the “likability” of self-confessed yeller and infamous grumpy guy Sen. Bernie Sanders (I-Vt.) never gets discussed. When will he go through the vetting that we expect of top-tier candidates?

Sanders is throwing elbows now, accusing Biden of vulnerability because he voted for the original North American Free Trade Agreement. But is Sanders really in a position to cast stones on electability? The self-declared socialist is an easy target for President Trump. In recent polling in Florida and Virginia, Sanders loses in a head-to-head match-up against Trump.

Sanders theorizes that somehow he will ignite such enthusiasm that he will beat Trump. But there is no evidence to support that notion. To the contrary, he risks losing moderate independents and Republicans who might choose to stay home rather than vote for a socialist over Trump.

Somehow, Sanders never takes as much as he gives out. Imagine if Hillary Clinton had a heart attack, promised to release all of her medical records and then only put out letters from her doctor? She would have been under assault for weeks. That’s what Sanders did, but neither his opponents nor the media have pressed him on full disclosure of his health records.

Warren was forced to put out a funding plan for Medicare-for-all. Sanders told my Post colleague Robert Costa he does not have to. “I don’t give a number," Sanders said, “and I’ll tell you why: It’s such a huge number, and it’s so complicated that if I gave a number you and 50 other people would go through it and say, ‘Oh . . .'”

That is an outrageous answer. Frankly, declaring that he will bring us Medicare-for-all without specifying how much it costs and grappling with its political unacceptability is not all that different from Trump saying in 2016 that he had some magic health-care plan he would reveal after getting elected.

Sanders seems to acknowledge that his health-care plan is so costly that people would ridicule it and accuse him of pulling the wool over voters’ eyes. That’s not a valid excuse for refusing to level with voters; it is confirmation that he feels exempt from demands for candor and transparency that other candidates face.

In sum, if Sanders is as viable as his supporters claim and as fund-raising totals suggest, he needs to get the same scrutiny that all top-tier contenders do. That means coming clean on his health records, spelling out his funding schemes, addressing whether he would run for a second term, explaining his tepid support for Hillary Clinton in 2016 and setting out a real foreign policy that is not merely a leftist version of Trump’s “America First.”

If Warren has any hope of recovering from her polling and fund-raising declines, she might want to try pressing Sanders at the next debate. Moderates who understand what a disaster it would be to nominate a self-declared socialist might want to start pressing him to explain why he will not be the American equivalent of Jeremy Corbyn, who ran the British Labour Party into the ground. And, unless the media want to repeat the error of 2016 in failing to seriously vet an improbable front-runner, they might want to begin, however late, their own due diligence.

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