Obama has given an occasional nod to the subject, promoting wind and solar power and mentioning climate change in his acceptance speech to the Democratic National Convention and during a campaign event last week in Miami, where he spoke about the threat of rising seas.

“That is not a joke. That is not a hoax. That’s our children’s future. Folks here in Miami understand that better than anybody,” Obama said. “The impact of climate change will be significant on our kids and grandkids unless we take those steps. We cannot just deny our way out of those things.”

For Republicans, skepticism about climate science, particularly the role humans have played in climate change, has become a litmus test. Romney has frequently mentioned his uncertainty. In his nomination acceptance speech, he used Obama’s lofty 2008 pledge “to slow the rise of the oceans and heal the planet” as a punchline, saying his goal was simply “to help you and your family.’’

Lisa Hoyos, co-founder of the Climate Parents, a recently formed California-based advocacy group, blames oil and energy companies for financing campaigns to attack climate science, efforts that have confused the public and given both parties political cover to back expanded oil drilling and coal use over alternative fuels.

Though Monday’s debate at Lynn is supposed to be focused on foreign policy, activists argue that the far-reaching implications of climate change, which could shrink food and water supplies and force massive population movements, deserve to be part of the discussion. With less than three weeks before the Nov. 6 election, it may be their last chance to put it on the national agenda.

Harold Wanless, a University of Miami geology professor who has documented decades of sea level rise in South Florida and signed the letter sent last week to both campaigns, said he’s frustrated by Washington politicians who took the issue seriously four years ago but now want to avoid it.

“I know we are worried about 10 million other things but this is the one that is going dominate our next century,” he said.