In Iowa, Representative Bruce Braley, the Democratic Senate nominee, has adopted the same approach as Mr. Udall. “Reducing our carbon output is not only necessary for the health of the planet, it’s an opportunity to continue to improve the health of the Iowa economy — which is and will remain my No. 1 priority,” Mr. Braley said.

In other states with competitive Senate races, such as Michigan, Democrats say growing public support for action to curb climate change — coupled with pronouncements by Republican candidates that human activity is not contributing to it, or their denials that the world is growing warmer — could help Democrats this year. They say it will definitely make the party stronger heading into the 2016 presidential election.

“Denying the existence of climate change is proving to be a problem for many Republicans right now, and certainly a long-term albatross for the party,” said Matt Canter, the deputy executive director of the Democratic Senatorial Campaign Committee. “Republican efforts to squeeze political gain out of this have fallen short.”