Audrina Patridge is backing Spencer Pratt‘s mayoral run … saying she knows he’ll deliver on his promises.

We caught up with “The Hills” star at Calamigos Resort in Malibu Friday night … and, we asked her about her former costar’s run for L.A. mayor.

Patridge says, once she realized he wasn’t joking, she was downright impressed with Spencer’s campaign … adding she knows firsthand how determined he is once he puts his mind to something.

As you know … Spencer’s taking on incumbent L.A. Mayor Karen Bass — and, he’s confident he’ll win this election.

While some — like Patridge and and Lakers Governor Jeanie Buss — have shown support for Spencer, others are less sure.

Pratt’s sister, Stephanie, warned people not to vote for him … and, his former costar Lauren Conrad responded awkwardly when asked about the run.