standards over the years, and say its reputation for polluted air presents an opportunity to make the case for government regulation.

But the Center for Regulatory Solutions, an industry group that's been at the forefront of the fight against the ozone rule, has repeatedly highlighted and targeted Denver in its campaigns, characterizing the advocacy as amplifying local opposition.

"Moving the standard down to where the far left wants it to be has received significant pushback in Colorado," said Matt Dempsey, spokesman for the Center for Regulatory Solutions (CRS).

"That's why you see such strong pushback from state, local and county officials, and elected representatives, because they have been on top of this issue."

The National Association of Manufacturers (NAM), the American Petroleum Institute (API) and others have also focused significant efforts onDenver, both to pressure local leaders to oppose the rule and to show national leaders the area's pain.

The EPA's rule from last year lowered the acceptable amount of ozone in ambient air to 70 parts per billion, from the 75 parts per billion set in 2008. Ozone is the main component in smog, and is linked to respiratory ailments like asthma.

Areas that exceed the standard have to come up with ways to clean their air. That usually means reducing the pollutants from fossil fuel burning that turn into ozone, something that could hurt the energy industry and sectors that rely on energy.

In December, the EPA said Denver is unlikely to meet the new standard by 2025, the only area outside of California to get the honor. In its