The debate over banning fracking in Colorado presents an “existential threat” to the game-changing oil and gas drilling method that has made the U.S. a top energy producer in just a few years, Sen. Cory Gardner said Thursday.

The Colorado Republican discussed the issue at the Colorado Oil and Gas Association’s 28th annual energy summit that wrapped up Thursday in Denver. Environmental groups are pushing a number of state ballot initiatives to give local authorities the power to ban fracking.