Stephanie Pratt Changes Tune on Brother Spencer Pratt’s Mayoral Campaign

Spencer Pratt's mayoral campaign just scored a shocking endorsement -- from someone who once called voting for him "stupidity" ... his own sister, Stephanie Pratt.

Stephanie's now she's doing a full-on political U-turn ... admitted she was the first person telling voters they were idiots if they backed her brother, before confessing, "Wow, was I wrong."

She told Vanity Fair Spencer's been grinding nonstop since the L.A. fires -- claiming "he has spent every day since the fires, finding the facts, the mistakes, the negligence and uncovering the truth that they never wanted us to know."

Pretty wild turnaround considering back in February she flat-out warned people not to vote for him ... saying L.A. didn't need "another unqualified and inexperienced mayor."

But hey -- Spencer's apparently won over one of his toughest critics ... and given the siblings' famously rocky relationship, that's saying something.