"Little new regulation is actually needed, merely enforcement of the existing law."
Maybe - but most of the Republican candidates for president and a great many Republican legislators favor - explicitly - abolishing the EPA entirely. And believe me - if the regulation authority disappeared, adherence to regulations, even the ones which are clearly a good idea, would also disappear. Capitalism has many wonderful aspects, but it does require regulation: A business segment can have 10 competitors, only one of whom is evil enough to pour choking smoke into the atmosphere - but as soon as its costs drop due to doing so, the other 9 have no choice but to follow suit or face inevitable (if, perhaps, gradual) demise. The job of regulation is not just to keep Evil Corporate Badness at bay, but to *allow* the majority of corporations run by good people in business.
Without some level of regulation, an ethical race to the bottom occurs; rational behavior for consumers individually is to seek low prices; rational behavior for businesses is to cut costs. Together those two rational positives add up to an irrational negative - of the type seen in Beijing, for example. The only way to prevent that is to apply an external force 'larger' than the two original ones. That's government's role. The right wing - or, what is now the right wing - in the US appears to believe, however, that at least in the case of environmental regulation, the government has no role whatsoever.
Yours is the false dichotomy - we are not in a situation where Democrats advise wild expansion of regulation and Republicans advise steady-state or slight tightening.
The alternative proposed by Republicans is in no way 'preventing the EPA from expanding'; characterizing it thus is like categorizing a single gunshot to the head as a diet plan.