Backyard boogie. Katharine McPhee serenaded Spencer Pratt during a fundraiser the singer and her husband, David Foster, threw for the reality star’s Los Angeles mayoral run. The couple threw a lavish fundraiser for the mayoral candidate in the backyard of their Brentwood Park home in Los Angeles over the weekend. Per a video shared on Instagram by news personality Alicia Summers, Foster, 76, played the keyboard while his wife, McPhee, used new lyrics for Tina Turner’s hit “The Best,” singing that Pratt is “better than [LA Mayor] Karen Bass and [LA Councilmember] Nithya Raman.” Katharine McPhee (seen above with her husband and a friend) serenaded Spencer Pratt during a fundraiser at the singer and husband David Foster’s home. The “American Idol” alum (seen performing above) remixed Tina Turner’s hit “The Best,” singing that Pratt is “better than [LA Mayor] Karen Bass and [LA Councilmember] Nithya Raman.” Foster played the keyboard (as seen above) as his wife sang.…