Joe Biden's running mate showed up at the start of Brandy and Monica's showdown Monday, and she reminded everyone why it's so important to vote in the upcoming election.

Senator Harris says Brandy and Monica have used their voices in a powerful way throughout their careers ... and she says everyone else can make an impact with their voices by voting.

Ya gotta see the video ... Monica's shocked when Kamala comes on the screen, and Brandy does a little fangirling. The respect and admiration here are definitely mutual.

Kamala makes her voting pitch, then says she's ready to sit back, relax and enjoy Brandy and Monica's showdown ... and she's even gonna sing along in her Howard shirt!!!