Gov. John Hickenlooper's administration has joined a lawsuit, filed by an oil and gas trade association, seeking to invalidate Longmont's voter-adopted citywide ban on a key drilling technology.

In December, the Colorado Oil and Gas Association filed a lawsuit opposing the ban on hydrofracturing, or "fracking," in Longmont.

Based on a motion by the association, the Colorado Oil and Gas Conservation Commission has been added to the suit in Boulder County District Court.

"The COGCC did not initiate this lawsuit or this process," Matt Lepore, executive director of the commission, said in a statement.

"That said, the COGCC does believe Longmont's ban on hydraulic fracturing is contrary to state law, and we believe clarity from the courts on this matter is important," Lepore said.

Fracking pumps large volumes of water, sand and trace chemicals into a well under pressure to crack rock and release oil and gas.

Longmont voters passed a resolution last November, with a 60 percent yes vote, to change the city charter to prohibit fracking.

Nearly all wells in Colorado are fracked, according to state data.

The state is already suing the city for its oil and gas regulations, which include limits on fracking, contending that only the state can regulate drilling siting and operations.

The suit challenging the rules was filed last July. At the time, Hickenlooper said Longmont's action would create pressure for other local governments to adopt their own regulations and "stir up a hornet's nest."

"It is consistent," said Bruce Baizell, energy program director at the environmental group Earthworks. "I am surprised that the governor would go after a democratically voted-on resolution."

Rigo Leal, a spokesman for Longmont, said the city "does not comment on ongoing litigation."

COGA also declined to comment on pending litigation, Doug Flanders, the association's polic director, said in an e-mail.

Mark Jaffe: 303-954-1912, mjaffe@denverpost.com or twitter.com/bymarkjaffe