LONGMONT, Colo. — This old farming town near the base of the Rocky Mountains has long been considered a conservative next-door neighbor to the ultraliberal college town of Boulder, a place bisected by the railroad and where middle-class families found a living at the vegetable cannery, sugar mill and Butterball turkey plant.

But this month, Longmont became the first town in Colorado to outlaw hydraulic fracturing, the oil-drilling practice commonly known as fracking. The ban has propelled Longmont to the fiercely contested forefront of the nation’s antifracking movement, inspiring other cities to push for similar prohibitions.

But it has also set the city on a collision course with oil companies and the State of Colorado.

“People really didn’t think through this too well,” Mayor Dennis L. Coombs said, sounding weary at the prospect of an onslaught of lawsuits. “We are where we are. I guess you have to respect the people.”

In a way, Longmont’s fracking ban is in a position similar to Colorado’s ballot measure legalizing small amounts of marijuana for recreational use. Both are lessons in the promise and peril of populism: both initiatives sailed through on Election Day despite opposition from the authorities, and both now face legal scrutiny and fights at all levels of government.