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 anti-fracking 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.

Gov. John W. Hickenlooper, a Democrat, has warned Longmont residents that the ban is likely to mean a lawsuit from the state, which insists that only it has the authority to regulate drilling. Already this summer, Colorado sued Longmont over earlier city rules that limit drilling near schools and homes.

Local leaders are also bracing for more lawsuits as they tell energy companies they can no longer frack their wells — a process that involves injecting thousands of gallons of pressurized water, sand and chemicals deep into the earth to fissure the rock and extract the oil and gas locked inside.

The ban does not outlaw all drilling, only the specific practice of hydraulic fracturing within the city limits, as well as the storage and disposal of waste created by the process.

"We’re going contrary to state laws," said Bill Swenson, one of seven former mayors of Longmont who fought the ban. "We are, in effect, taking your property."

Fracking has allowed drillers to unlock huge new reservoirs of oil and natural gas over the past few years, and has kick-started economies from North Dakota to western Pennsylvania to here in northern Colorado. The industry says the practice is environmentally safe, but opponents have raised concerns about water contamination and air pollution while objecting to islands of well pads and forests of drilling towers in their communities.

The Colorado Oil and Gas Association, the main lobbying group for the energy industry here, criticized the ban as confrontational and encroaching on the private property of companies that have rights to oil and gas buried deep beneath Longmont’s streets, parks and reservoirs.

"Are the taxpayers of Longmont prepared to provide fair compensation to all of the oil and gas lease holders in Longmont?" said Tisha Schuller, the group’s president.

Supporters of the ban call it a "citizen uprising" against a rush of drilling that has spread like brush fire through towns across the plains of northern Colorado.

In nearby Firestone, wells sit within a few hundred feet of libraries, schools and subdivisions. In Greeley, herds of tanker trucks line up at city fire hydrants at dawn to load water for fracking. Earlier this year, a federal scientist reported finding elevated levels of propane and benzene in the air around Erie. City officials and environmental advocates have even led fracking tours of communities where drilling is at its peak.

When people learned of plans to sink wells in Longmont near the Union Reservoir and a playground and recreational area on the east end of town, a response began to coalesce: not here. Supporters said the state’s decision to sue over Longmont’s regulations stiffened their resolve.

At the start, the ban seemed like a doomed idea.

The energy industry poured money and resources into fighting it, raising more than $500,000 to send out mailers and buy advertisements saying the ban would drive away businesses and incite expensive court battles. The major newspapers in Denver, Boulder and Longmont all urged voters to reject the proposal.

"I had no idea we could upset an entire state government and a trillion-dollar industry," said Michael Bellmont, an insurance agent who helped gather thousands of signatures and knocked on doors to persuade voters.

Advocates of the ban focused less on climate change and environmental concerns than on hitting voters where they lived: Do you want oil wells venting near your back yard? Do you want drilling near your schools?

The industry said the arguments were based on fear-mongering, deception and anti-fracking hysteria, but they resonated with voters. The ban passed 60 percent to 40 percent, with broad bipartisan support.

One recent afternoon, a few supporters who helped get the ban passed drove through town to visit some of the "red sites" — areas that had been leased for drilling, or could be in the future. They drove past public parks, open spaces and golf courses and stopped at the Union Reservoir, still and limpid under a cloudy sky.

"There’s a swim beach, there’s sailing, and there will be eight well pads," said Kaye Fissinger, a supporter of the ban, pointing out potential drilling sites in the distance. "You come out here to relax. You don’t come out here to have your air polluted."

© 2012 The New York Times Company