CHARLOTTE, N.C. — It was just six words, but when President Barack Obama gave a shout-out to global warming in his acceptance speech this month, he reintroduced an issue that had all but disappeared from the political debate.

“Climate change is not a hoax,” Obama said, an assertion that brought Democratic National Convention delegates to their feet, as he pledged to continue approaching energy policy in a way he said would “continue to reduce the carbon pollution that is heating our planet.”

In a year when the political debate has lacked nearly any discussion of climate change, some environmentalists have struggled to summon enthusiasm for the Democratic president they helped elect in 2008 in part because of his views on global warming.

So they rejoiced when the president rebutted a taunt tossed out by Republican candidate Mitt Romney the week before. Romney had quipped in his acceptance speech in Tampa, Fla., that Obama “promised to begin to slow the rise of the oceans and heal the planet.”

“My promise is to help you and your family,” Romney added.

It was a rhetorical flourish, an attack line offered to make the point that Romney understands the kitchen table issues that, he says, the president doesn’t. But environmentalists heard it as heresy.

“Twenty years from now, history is going to judge the next generation on how they responded to the destabilization of our climate,” said Michael Brune, executive director of the Sierra Club. “With a couple of short sentences, Romney made clear what’s at stake in this election.”

Although climate change typically ranks below such issues as the economy, polling done in March 2012 by Yale University and George Mason University found that 72 percent of Americans think that global warming should be a priority for the president and Congress. Among registered voters, 84 percent of Democrats, 68 percent of independents and 52 percent of Republicans think global warming should be a priority.

Regardless of the candidates’ relative silence about global warming on the campaign trail, the next president will face tough choices on controversial energy and environmental issues such as whether to approve the Keystone XL pipeline and how to handle natural gas development and the environmentally fraught “fracking” that goes with it.

The silence on the campaign trail belies the reality — and the gravity — for many coastal communities. Planners in south Florida and New York City already are looking at the multibillion-dollar expense of upgrading infrastructure to address rising sea levels.

Romney has said previously that he believes climate change is occurring and that human activity is a contributing factor. During the Republican primary season, though, he said he didn’t believe it was the right course to spend “trillions and trillions” to reduce carbon emissions. More recently, he said in a questionnaire submitted to Science Debate, a nonprofit organization focusing on science issues in the presidential campaign, that he believes human activity contributes to global warming and that policymakers should consider the risk of negative consequences.

Frank Maisano, a lobbyist whose firm represents energy interests and who has been involved in climate change discussions for 15 years, cautioned not to read too much into Romney’s dig about the rise of the oceans. It was designed to show Obama is “a little bit out of touch,” he said.

There are ways to make climate change part of the overall discussion on energy without actually saying “climate change,” said Sen. Mark Begich, D-Alaska, who hails not only from an energy-producing state, but one that has to grapple with the eroded coastlines, fiercer storms and melting icepack caused by the changing climate.

Rather than talking about climate change in a vacuum, Begich said, Democrats need to talk about it in the context of national security, energy security and a thriving green economy.

“I think we have a habit as Democrats, we want to get touchy feely and policy wonk stuff to death,” he said, while speaking at an energy panel at the convention. “Instead, just plain and simple, this is about economic security, it’s about national security. We cater so deep to our base, we forget the average person is looking at, well, how does it affect them? What does it do for national security? What does it do for the economy?”

Plenty is at stake, said Marc Weiss, a documentary filmmaker and Obama supporter whose film on the history of the environmental movement will be released in the coming month. He continues to support the Obama-Biden campaign. But Weiss wants more voices to speak out on climate.

“It’s not just about the moral imperative,” he said. “I think it could be a winning political strategy. There’s a dramatic choice here, and for me it’s a no-brainer.”