"People who deny the existence of climate change or want to try to run suggesting that they aren't scientists, saying they don't get it and they can't really say what's going on around them. ... I think that's the losing side of the argument," he said. "I'm certain that if you think about this in the cycles going forward, anybody who tries to run against this in 2016 is going to have a hard time running on that platform."
"If you're a climate denier trying to run nationally," he added later, "I think you have a very hard road."
He did acknowledge, though, that Democrats in coal-producing states -- which include Senate candidates like Kentucky's Alison Lundergran Grimes and West Virginia's Natalie Tennant, who have criticized the rules -- have a more difficult task ahead of them in 2014.
"There's no doubt there are some states where this is an issue that presents... political challenges," he said. More broadly, he added, Democrats are "going to have to make their own case to their constituents."
Photo: Michael Bonfigli/The Christian Science Monitor.