With just days until the US presidential vote, the devastation caused by megastorm Sandy has re-injected climate change into a neck-and-neck campaign that had largely ignored it until now.

Sandy killed at least 95 people in the US alone and wrought havoc through the country's northeast, prompting questions over what impact global warming may have had in creating or intensifying the dangerous storm.

On Thursday, New York City Mayor Michael Bloomberg, a political independent, gave a last-minute endorsement to President Barack Obama for re-election, after withholding his support from both candidates four years ago.

In an op-ed carried on the wires of his eponymous news agency, Bloomberg said he was backing the president because Obama was best placed to lead the fight against global warming.

"Our climate is changing. And while the increase in extreme weather we have experienced in New York City and around the world may or may not be the result of it, the risk that it may be -- given the devastation it is wreaking -- should be enough to compel all elected leaders to take immediate action," he wrote.

In a statement thanking Bloomberg, Obama called climate change one of "the most important issues of our time," saying it is "a threat to our children's future."

But Republican challenger Mitt Romney Thursday maintained his silence on the subject when speaking of Sandy's victims -- even after a heckler pressed him on the subject during a rally in Virginia.

"What about climate? What about climate? That's what caused this monster storm," the man shouted, brandishing a sign reading "End Climate Silence."

He was immediately booed by the crowd, who ejected him to chants of "USA! USA!"

Just a week ago, the issue of global warming was nearly completely off the campaign radar, Alden Meyer, head of the "Union of Concerned Scientists," told AFP, noting that not a word was mentioned on the subject during three presidential debates.

"Sandy has raised the stakes on this issue in the campaign and you see this now with the Bloomberg announcement," he said.

For many Americans, said Heather Taylor of the Natural Resources Defense Council, climate change worries just can't compete with concerns over the troubled economy.

"There is no doubt that for Americans, climate change is not a high priority, and that is problematic. Our job is to connect the dots," she said.

According to a recent poll by the Pew Research Center, an independent organization, less than half of Americans -- 42 percent -- believe that global warming is "mostly caused by human activity."

The Sierra Club's Dave Hamilton blamed news media, in part, for the lack of public interest, saying news agencies are devoting less and less space on the issue.

According to the DailyClimate.org website, the number of articles discussing climate change has shrunk by 41 percent between 2009 and 2011, he said.

Manik Roy of the NGO "Center for Climate and Energy Solutions," argued that "once the Senate failed to pass the climate bill everybody stopped talking about it: politicians, business leaders, even most of the environmental groups in the US."

The 2009 climate bill included a mechanism to create a market to sell credits for emissions of carbon dioxide, one of the principal greenhouse gasses, aiming to make it more expensive for businesses to pollute and entice them to invest in cleaner fuels.

But Frank Maisano of the firm Bracewell and Giuliani, a law firm that represents energy companies, said tying Sandy's destruction to climate change was an oversimplification.

"No scientist is making this case -- even the ones who are prone to be in favor of that are not going there," he argued.

Moreover, he said, ending US reliance on fossil fuels would be a massive undertaking for a country that relies on coal to generate nearly 40 percent of its electricity.

"Even in good times, climate change is a low priority issue. And in time of economic distress it becomes even a lower economic issue," he told AFP.