In 2009, the Obama administration’s environmental team called a group of climate activists to the White House to deliver a message: Climate change doesn’t sell and only provokes economic attacks from the right that are too difficult to counter. As former Vice President Joseph R. Biden Jr. prepares to assume the Democratic Party’s presidential nomination, the changing climate is now a core campaign issue — and a focus for fund-raising. Plans for tackling rising global temperatures will be in the spotlight Wednesday at the Democratic convention. And Mr. Biden has raised more than $15 million in candidate contributions from hundreds of new donors who specifically identify with climate change as a cause.

That climate-specific fund-raising may make up just about 5 percent of the total he has raised so far. It’s dwarfed by fossil fuel donations to President Trump, who took in $10 million from a single fund-raiser in June, held by the oil billionaire Kelcy Warren, and whose super PAC, America First Action, has seen millions pour in from coal and oil moguls, according to the Center for Responsive Politics, which tracks campaign donations. It is not known how much unregulated money is going to super PACs aligned with Democrats from other self-identified climate donors.

“That is a sea change. We’ve now got a class of people called ‘climate donors’ in a way we had environmental donors before,” said David Bookbinder, general counsel for the Niskanen Center, a nonpartisan think tank in Washington.

“Climate has taken over as an issue on its own. People are finally understanding that we have a truly existential crisis on our hands,” Mr. Bookbinder added. Publicly embracing climate change solutions was viewed as a political liability, as recently as a decade ago, he said. During Barack Obama’s re-election run in 2012, the issue was hardly mentioned.

Now donors are sending a new message: “We want to make it easy to do the right thing. We should reward campaigns and candidates for having the right policies,” said Matt Rogers, a co-founder of the digital thermostat company Nest.