Katharine McPhee and her husband David Foster teamed up to stump for Spencer Prattand 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.”