@hawkeye10,
I don't pretend to be all that knowledgeable about this, but as I understand it...
The Clean Air Act was signed into law in 1970 - when Nixon was President. It gave the EPA the authority to regulate emissions. The law was amended a couple of times and was decreed to be constitutional in 2007.
Congress, because of regional differences rather than party affiliation, has not tried to strip the EPA of its authority.