In October, Sen. Bernie Sanders (I-Vt.), at 78 the oldest Democrat in the field, had a heart attack. He promised to release his full medical records before primary voting started. Now, on the eve of the second presidential nominating contest, Sanders still has not released his full medical records. He insists he is “in good health.” He told NBC’s Chuck Todd he has released as much medical information as other candidates, which ignores an important point: There is no other 78-year-old who had a heart attack just months ago in the field. He says if he released his records, there will be no end to it. He says his doctor told him to walk and sleep more. (Good thing he is not running for a job where sleep may be disturbed constantly due to national crises or jet-lagged travel.) So, naturally former vice president Joe Biden runs a hard-hitting ad … against former South Bend, Ind., mayor Pete Buttigieg. Thunk. The clear beneficiary is Sanders.

We remember all too well in 2016 that the traditional Republican candidates spent months beating up on one another as they vied to be the single one standing between Donald Trump and the nomination. As they bludgeoned one another, Trump ran away with the race.

Other than some swipes during the debates that running as a socialist will be the death knell of the party, Sanders’s competitors before Sunday had not done much to dent the Vermont senator, who has no problem going after Biden. It is not like there isn’t a wealth of material ranging from his broken promise to release his health records to his “no trade deal is ever good enough” stance.

Play the Post Opinions Simulator to see what might happen in the Democratic primary.

On Sunday, we finally saw Biden launch direct attacks on Sanders’s refusal to explain how to pay for Medicare-for-all, and Buttigieg raised the electability argument, making the case it will be very hard to win with a self-labeled socialist.

Democrats cannot afford to repeat the Republicans’ mistake. If Biden continues going after Buttigieg, Buttigieg goes after Biden, and Sen. Amy Klobuchar (D-Minn.) goes after Buttigieg. Sanders will be the beneficiary. It is natural to go at the person directly ahead of you in the polls, but you cannot let the guy leading in a state get a pass. The combatants will depress each other’s share of the vote, and Sanders will sail on to victory. Over the weekend, we finally saw sustained attacks directed at the front-runner in the state, Sanders.

Rather than let Sanders build up momentum and delegates, the candidates should step up their criticism of Sanders and begin earnestly vetting him now. Klobuchar, for example, could point out that the last guy who promised to release sensitive information but never did was Trump. Biden, instead of trying to clobber Buttigieg for lack of experience (as if the voters didn’t know his highest political position has been mayor), should start hammering Sanders, who has been in Washington for decades and never accomplished what Biden and President Barack Obama did on the Affordable Care Act.

Here is where Mike Bloomberg and his millions could serve the party and the country. The former New York mayor has virtually unlimited funds, so he surely could start pointing out that down-ticket Democrats are endorsing him, not the socialist who will drag them down. Bloomberg could release every scrap of his medical history and then dare other contenders including Sanders to do the same. If he focuses purely on Trump, Bloomberg will be caught in the same trap as Sanders’s other competitors. Bloomberg should not let Sanders pick up a head of steam before Bloomberg even starts competing in primary contests.

Democrats are right to be worried that Sanders might get the nomination and then hand up four more years of Trump. They should intensify their efforts do something about it before it is too late.

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