That means Ayotte could be pulled by competing interests—pressure to act on climate change while opposing a massive regulatory scheme—that will make it difficult to simply reject the president's agenda.
"If you're going to oppose something then the question becomes, 'OK and so what is your plan then?' That's something that voters expect and that should be addressed," Republican pollster and strategist David Winston said. "Otherwise it looks like you're just saying no."
Senator Mark Kirk of Illinois voted to effectively block the power-plant regulations in June, a move that drew sharp criticism from green groups and sparked an ad campaign paid for by the Natural Resources Defense Council attacking Kirk's environmental record.
Kirk did not comment on the final version of the regulation on Monday when asked in the Capitol. An aide said that the senator was in a rush to get to votes. That vote was on the motion to proceed to legislation to defund Planned Parenthood, another problematic vote for Kirk in Illinois. In this case, he was the only Republican to break with his party and vote against the cloture motion.
If Kirk rejects the final EPA regulation, because he has also expressed concern over the threat of global warming, he will have to explain the vote and may be called upon to propose an alternate policy.
On the other side of the aisle is Michael Bennet, a moderate Democrat who faces pressure from his party to protect the president's climate agenda while at the same time pleasing the fossil-fuel industry in his state.
Natural gas is big business in Colorado, a state that has seen a boom in jobs from fracking. But the final version of Obama's climate agenda stands to give clean energy more of a boost than natural gas relative to an earlier version of the rule, a move that provoked backlash from the industry on Monday.
But Colorado may be better prepared than most fossil-fuel states to meet the standards, given the growth in wind and solar energy there thanks to a 30 percent renewable energy standard. And that's the industry Bennet, at least in his initial response, is leaning on.
In a statement, he said the state was "well-positioned" to meet the federal targets, adding, "Colorado's leadership on this issue is already paying off."
Eric Sondermann, an independent political consultant in Denver, said that Bennet is likely to sidestep some of the divisive energy politics that sprung up in Udall's 2014 race, making it easier for him to come out in support of the Clean Power Plan.
"He's under pressure to be reasonable, to be a moderate, and not to be a zealot on the environmental left," Sondermann said. "But that's the clothing that tends to fit Michael Bennet anyway. At the end of the day, Bennet is going to side with the president on this, but his manner will give lip service to concerns about jobs and those impacted by the regulations."