Spencer Pratt is using Khloé Kardashian to bash Los Angeles Mayor Karen Bass ... editing an old clip from KK's reality show to call his political opponent a liar.

In a new attack ad for his mayoral campaign, Spencer turns to "Keeping Up With The Kardashians" ... Khloé's watching her phone and screaming "liar" as Bass talks about the wildfire that destroyed Spencer's Pacific Palisades home.

The 'KUTWK' is from an old episode where Khloé bashes her ex Tristan Thompson ... but the Bass clip is new -- it's from last week's televised debate between Spencer, Bass, and Nithya Raman.

Spencer posted the doctored video on social media with the caption, "Everyone agrees: Karen Bass is an incredible liar." Check out the clip above from when Spence popped into "TMZ Live" yesterday to tell us more about his POV on his opponents.

Needless to say, we are Keeping Up With The Pratt Mayoral Campaign!