Poorer nations’ demand that wealthier countries help bankroll their transition to cleaner energy has emerged as the biggest obstacle for world leaders seeking to reach an agreement on emissions at a summit that begins this month.

World leaders arrive in Paris in late November with the aim of negotiating an agreement to slash carbon dioxide emissions beyond 2020. But many developing countries say highly industrialized nations haven’t yet pledged enough to help them transition to lower-carbon energy or to prepare for the effects of for a more adverse climate.

“Finance is a very big issue in this negotiation,” said Todd Stern, the U.S. special envoy for climate change, in an interview.

The latest round of climate negotiations organized through the United Nations was meant to blur the line between highly industrialized, richer countries that have burned large amounts of fossil fuels for more than a century and the poorer, developing countries that still rely on burning more coal and oil to achieve rapid growth and lift hundreds of millions out of poverty.

Rather than assigning across-the-board reductions in emissions of greenhouse gases, the Paris talks allow countries to set their own emissions targets based on their particular level of development. That approach will prevent the Paris accord from limiting global warming as much as officials had sought, but it may allow for a politically palatable agreement that brings China and other big emerging markets to the table to limit emissions down the road.

Still, the split between richer and poorer countries persists in the area of finance: The participation of many developing countries—including their decision on whether to sign on to the agreement next month—is based on richer countries following through with plans to mobilize money for solar and other forms of green energy, and to compensate poorer countries for their adjustments to a harsher climate.

“One of the pieces that is very, very critical for this process is to provide that transparent support to developing countries,” said Christiana Figueres, head of the United Nations Framework on Climate Change, in an interview in Washington. “But it can only be transparent and quantifiable if everyone agrees on the methodology.”

The stated goal is for richer countries to kick in $100 billion a year of their own money and related private financing for developing nations by 2020. A report last month from the leading group of developed countries—the Organization for Economic Cooperation and Development—estimated that more affluent countries delivered $61.8 billion in financing last year to help developing economies limit carbon emissions and fend off extreme weather and rising sea levels.

Ms. Figueres and other leading officials say the figures from the OECD show progress, but not all developing countries are satisfied.

“Those numbers haven’t been consulted with or accepted by developing countries,” said Ms. Figueres, a Costa Rican diplomat based in Bonn, Germany. In its report, the OECD acknowledges compiling the financing figures was “technically difficult and politically sensitive issue.”

Under the consensus-based approach of the climate talks, nearly 200 countries have to agree on the text to launch the accord.

“The big threat is that there’s a bloc of countries that basically says, ‘No, no, wait a second, we believe the whole climate-finance system is inadequate,’ ” said Paul Bledsoe, a former climate official in the Clinton White House and consultant in Washington. “And they make a big demonstration, and they walk out of the meetings.”

The finance problem is politically sensitive for poorer countries in part because they will actually need trillions of dollars overall to finance energy transitions and prepare for new climate realities, Ms. Figueres said. One of the top goals of the Paris agreement is to produce a convincing blueprint for world energy use decades into the future to help mobilize private funding for greener alternatives, essentially redirecting money currently invested in fossil fuels, she said.

The Obama administration, for its part, has pledged to put $3 billion in the Green Climate Fund, the leading fund for aiding developing countries. “We think it’s the right thing to do to provide climate finance, and we’ll continue to do that with other donor countries—developed countries—but also some developing, ” Mr. Stern said. Chinese officials announced in Washington in September that they would contribute $3.1 billion to other developing countries to combat climate change.

But critics of the Paris talks, including Republicans in Washington, warn that reducing the use of coal and other high-carbon fuels is expensive enough domestically, and some say they will work to block the Obama administration’s $3 billion aid pledge.

“Developed countries do have a commitment to continue to provide finance, and we have every intention of doing that,” Mr. Stern said. The issue will be at the forefront in preliminary talks in coming days that are meant to prepare for the Paris summit later this month.

Besides grants and loans, developing countries are looking for help with everything from advice on infrastructure to compulsory licensing of energy-related patents held in highly industrialized countries, according to people following the talks.

“They’re looking for capacity building, they’re looking for technology transfer, they’re looking for finance,” said Jennifer Morgan, director of climate and energy at the World Resources Institute, an environmental think tank.