Critics charge the section, known as new source review, has been targeted by the White House ever since Vice President Cheney issued his 2001 energy report.
Lawsuits against the government already are being threatened by opponents, who suggest the devil of the plan is in the details.
This would mean power plants making changes to generators, boilers or turbines would not have to be done with modern pollution control technology.
Critics, like the Environmental Defense Fund, are calling the measure an excuse to pollute. Officials with the group suggested that, in theory, a company that has not reached pollution limits could replace equipment that allows a few more contaminates into the air yet remain legal.
For their part, EPA officials are stating that the new rule will not increase pollution levels.