In the left-leaning Big Apple, it’s a fashion faux pas more fatal than walking around in sandals with socks, or strapping a fanny pack around your waist: wearing a “Make America Great Again” hat.

To see for myself, I sported the fire-engine-red baseball cap worn by Donald Trump on the campaign trail in liberal gin joints and shops across Manhattan and Brooklyn.

I may as well have been wearing a Red Sox hat at Yankee Stadium.

The mere sight of my cap nearly caused a riot at the historic Stonewall Inn on Christopher Street — site of the 1969 riots that launched the gay rights movement.

“You come into a gay bar — THIS gay bar — with THAT hat!” one woman lectured as a large crowd gathered.

At Soho’s sceney La Esquina, where celebs like Julia Roberts nosh on $26 enchiladas, servers nearly lost their lunch when I showed up.

“Oh my God, do you see that? Is he serious? Is he kidding me?” one waiter gasped.

My companion and I were quickly shunted to an out-of-sight table near a back wall.

At Sylvia’s soul food restaurant in Harlem, my server, Patrick Bros, admitted after my meal that he was taken aback by the hat, but figured, “Whatever.”

Nearby, folks were less diplomatic.

“Don’t talk to him!” a man instructed a street vendor as I browsed along 125th Street near the Apollo Theater.

Hipsters and trustafarians along Bedford Avenue in Williamsburg either did a double take, or shot me a death stare or a snarky remark.

“Take off that stupid f—ing hat!” one skinny-jeans wearer sneered.

At high-end chapeau peddler Goorin Bros., I overheard a salesman tell his colleague, “I’m losing my sh–!” as I walked in. When I asked him to hold my hat while I admired a fedora, he grimaced.

“I’m surprised nobody’s knocked that hat off your head!” a mother of two scolded me as we crossed paths along Central Park West and 63rd Street. “Make America Great Again — right!”

On the sidewalk near Lincoln Center, I spotted comedian Chris Rock and asked him to join me in a selfie. He raised his palm, and kept on walking.