annual report on air pollution released in April, the American Lung Association, which supports the EPA's rule, ranked the Denver area as the 8th worst for ozone in the country, saying its air improved since last year, but is still worse than two decades ago.

Due to its manufacturing base, the oil and gas boom, population growth and other factors, Denver produces many of the pollutants that become ozone. But owing to the Rocky Mountains, wind patterns and similar factors, the city has had difficulty cleaning its air.

"There are a number of drivers for ozone in the air, and so far we haven't gotten a handle on how to ratchet that back and come into compliance with even the old standards, or even the new standard," said Justin Pidot,Kyle Saunders, a political science professor at Colorado State University.

There are currently five Republicans vying to face Bennet, and none have made ozone a major point of their campaign yet.

Public health and environmental advocates say the energy industry has it all wrong with its Denver strategy.

"The polluters may well be looking at Colorado as their Alamo, the place to take their last stand," said Pete Maysmith, executive director of Conservation Colorado. "Because once a clean air policy or regulation is moving forward in Colorado, which is a purple state politically and has a heavy and significant fossil fuel industry presence, it's kind of game over elsewhere."

Maysmith, who grew up in Denver and has numerous stories going back years about the city's thick, brown air, labeled the efforts to