TENSIONS IN the Democratic presidential race exploded in Nevada over the weekend. Supporters of Sen. Bernie Sanders (I-Vt.) shouted, cursed and threw chairs during a state party convention in which they failed to force rules changes they wanted. Even though they were attempting to get more delegates than the caucus results in the state suggested they deserved, they attacked the process as unfair. The state party chair subsequently received death threats against her and her family.

Mr. Sanders responded with self-righteousness and hypocrisy. He released a statement in which he listed a series of procedural complaints about the Nevada convention, attacked the Democratic Party for not being inclusive enough and warned that “millions of Americans are outraged” and that “the political world is changing.” He offered a throwaway line, three paragraphs down, condemning his supporters’ hooliganism in a statement that mostly justified it.

Mr. Sanders’s irresponsibility is sadly unsurprising. He has stirred up populist energy over the past several months with anti-corporate scapegoating and extravagant claims about policy. He has indulged and encouraged hyperbolic feelings that the country is badly adrift, that most of the nation agrees with a left-wing agenda but is trapped in a corrupt system, and that nothing but a political revolution will do. He has attracted some big, passionate crowds. But as he has lagged in votes, he increasingly has questioned the legitimacy of the process and encouraged his supporters to feel disenfranchised. The result is a toxic mix of unreason, revolutionary fervor and perceived grievance.

What is particularly galling about the Sanders camp’s complaints of disenfranchisement is that Mr. Sanders has benefited or tried to benefit from a variety of sketchy quirks of the nominating process. He has claimed support for his cause in caucuses, which are quite exclusive, but he complains about closed primary elections, which are more inclusive. In Nevada, his supporters were trying to game the rules to get more delegates and got upset when they did not succeed. As veteran Nevada politics reporter Jon Ralston put it, “Despite their social media frothing and self-righteous screeds, the facts reveal that the Sanders folks disregarded rules, then when shown the truth, attacked organizers and party officials as tools of a conspiracy to defraud the senator of what was never rightfully his in the first place.”

Mr. Sanders denies reality when he tells supporters he still has a plausible pathway to the Democratic presidential nomination. But passion cannot trump reality. It also cannot excuse violence, threats and attempts at mob rule. It is past time for Mr. Sanders to be honest with his supporters, before they take the campaign’s irresponsible ethos to greater extremes and thereby help ensure the election of Donald Trump.