Democratic presidential debate (6 p.m., FNC): The nine candidates are scheduled to appear.
Rita (6 p.m., TCM): This documentary explores the fascinating life of screen siren Rita Hayworth.
Big Brother 4 (7 p.m., Ch. 2) and Cupid (8 p.m., Ch. 2): CBS has two of the best "reality" shows in "Survivor" and "Amazing Race," but that's no excuse for having these two, which are among the worst.
Whoopi (7 p.m., Ch. 5): Whoopi Goldberg stars in this opinionated-but-not-funny new sitcom.
Tactical to Practical (7 p.m., History): Former "Survivor" contestant Hunter Ellis (an ex-Navy pilot) hosts this new series about military innovations adapted to civilian use.
Happy Family (7:30 p.m., Ch. 5): See accompanying story.
The O.C. (8 p.m., Ch. 13): Kirsten's father and his young, gorgeous girlfriend visit, and the gal pal puts the moves on teenage Ryan.