Katharine McPhee and her husband David Foster teamed up to stump for Spencer Pratt and trash his political opponents as he runs for Los Angeles mayor ... they did it in song form, and it's all on video.

The "American Idol" alum grabbed the mic and started singing at a campaign fundraiser for Spencer at the home she shares with David, who played the piano as Katharine put a political twist on Tina Turner's "The Best."

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Check out the clip ... Katharine switches up the lyrics and sings, "Spencer you're simply the best / Better than all the rest / Better than Karen Bass / And Nithya Raman / You're stuck in our hearts / We'll hang on every word you say / We've been torn apart / He's gonna fix this broken L.A."

Spencer was in the house ... sitting just feet away from Katharine as she sang next to the pool in her backyard ... and those on hand ate it all up.

Katharine's among the celebs donating to Spencer's political campaign ... and he's reported to have raised more money than incumbent Mayor Bass.