Jonathan Cheban is riding hard for Spencer Pratt's mayoral bid ... and things got so chaotic when TMZ ran into him Monday, it ended with a stranger flashing him in the middle of Beverly Hills.

Check out the video ... Kim Kardashian's longtime pal Jonathan -- better known as FoodGod -- didn't mince words Monday when we asked about Karen Bass' swipe earlier in the day at Pratt's reality TV roots. Cheban argues Spencer could breathe new life into Los Angeles and even claims he'd consider moving back to the city if Pratt somehow pulls off the upset.

But before he could fully make his case, a passerby jumped into the conversation with an unrelated rant ... and that's when the interview took a sharp left turn.

Cheban wasn't about to let the interruption slide ... mocking the woman's purse as a fake and telling her to "put some clothes on" before urging her to buy some SKIMS. The woman then took things to another level by flashing Cheban and our cameraperson ... leaving Cheban stunned and joking that the bizarre encounter is all the proof he needs that L.A. is due for a shake-up.

Love him or hate him, FoodGod definitely wasn't lacking confidence when it came to Spencer's chances ... and by the end of the encounter, he was practically campaigning on Pratt's behalf.

One thing's for sure ... we didn't expect our run-in with Cheban to end quite like this!