Spencer Pratt might have a ton of supporters backing his Los Angeles mayoral run … but, his sister Stephanie isn’t one of them — because she’s warning people not to elect him!

Stephanie took to social media Saturday to bash her bro’s political ambitions … admitting he’s done great work for the Pacific Palisades community — but he can’t be responisible for 4 million citizens.

Spencer’s only running to boost sales of his memoir “The Guy You Loved to Hate: Confessions from a Reality TV Villain,” Stephanie claims … and, she tells people to just wait and see what he does with their tax dollars if elected.

Stephanie does write she’s worried about Los Angeles — referring to Mayor Karen Bass as “ASSBASS” in one post — but adds the city doesn’t need “another unqualified and inexperienced mayor. A vote for him is a vote for stupidity.”

Spencer announced his run for mayor last month … speaking mainly about the mismanagement by the local government of the Pacific Palisades Fire which burned his house among many other structures in the area.

We spoke to Spencer at a campaign event last week, and he’s confident he can defeat Mayor Bass — who he calls Karen Basura, trash in Spanish — mainly because he’s sure she’s going to end up in jail for obstruction of justice. Much of his comments seemed ripped straight from the President Trump political playbook

However, if he wants to move into the mayor’s residence, the Getty House, Spencer may need to address the discord in his own.