BOULDER, Colo.—This spring, city contractors will fan out across this well-to-do college town to unscrew light bulbs in thousands of homes and replace them with more energy-efficient models, at taxpayer expense.

City officials never dreamed they'd have to play nanny when they set out in 2006 to make Boulder a role model in the fight against global warming. The cause seemed like a natural fit in a place where residents tend to be politically liberal and passionate about the great outdoors.

Instead,...