At a Sanders Rally in Phoenix, a protester unveiled a Nazi flag and shouted, “Heil Hitler.”

Clearly, the problem is “Bernie Bros” saying nasty things on Twitter.

More seriously, why the hell aren’t Sanders and Biden getting Secret Service protection?

Moments after Sen. Bernie Sanders (I-Vt.) took the stage at his campaign rally in Phoenix on Thursday night, the crowd was on its feet cheering madly for the Democratic presidential candidate.

But those cheers were swiftly replaced by deafening boos when Sanders’s supporters noticed that one man standing behind the senator in an upper section of the Arizona Veterans Memorial Coliseum wasn’t waving a “Bernie” sign like many of those around him.

Instead, the man was holding a red flag above his head — and it was emblazoned with a swastika.

“It was absolutely wild,” Brianna Westbrook, a national surrogate for the Sanders campaign, told The Washington Post. “I never thought I would have seen a swastika at a political event. It’s gross.”

While people near the protester quickly ripped the offending item out of his hands and he was removed from the arena, the mere appearance of a Nazi flag at an event dedicated to a democratic socialist who could become the country’s first Jewish president sparked outcry. The moment, captured in videos and photos that circulated on social media Thursday night, was denounced as an act of anti-Semitism and prompted increased concerns about Sanders’s safety on the campaign trail.

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“I was expecting Trump supporters to be protesting. I didn’t expect a swastika flag to be unfurled,” Orlando Garrido, a rally attendee, told The Post. “I never thought I would actually see something like that.”

On Twitter, the display was widely condemned as observers pointed out that Sanders’s Polish relatives were murdered in the Holocaust.

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The Anti-Defamation League identified the protester Friday as Robert Sterkeson, a resident of Arizona. According to the ADL, Sterkeson is “a self-described ‘stunt activist’ who has harassed a range of Jewish and Muslim organizations and events.”