The alternate American pavilion, with its free espresso truck, tins of themed M&M’s and wireless internet that tells new users “the U.S. has not gone dark on climate action,” has rapidly become a hub of activity at the United Nations global warming negotiations taking place this week. On Saturday, a line of people waited in the rain to hear Michael R. Bloomberg, the former mayor of New York, Gov. Jerry Brown of California and a handful of United States senators, all Democrats, declare that much of America remains committed to reducing planet-warming carbon dioxide emissions.

“We’re in,” said Mr. Bloomberg, who put more than $1 million toward funding the pavilion, according to his office. “Just because the federal government has chosen not to participate,” he added in an interview, “the American public represented by its elected officials at other levels, by corporations, by universities, we understand that there’s a problem and we have to help solve that problem if we’re going to have a future in this world.”

The dueling American delegations here mirror a larger division within the United States over climate change. Mr. Trump’s decision in June to withdraw from the Paris agreement was popular with his supporters. On Thursday, the State Department and the Interior Department sent high-level political officials to address a conference in Texas sponsored by the Heartland Institute, which rejects the scientific consensus that climate change is occurring and primarily caused by human emissions. Scott Pruitt, the administrator of the Environmental Protection Agency, sent a video message of support.

“We have tremendous natural resources, from coal to natural gas to oil, to generate electricity in a very cost-effective way,” Mr. Pruitt told the conference. “We should celebrate that and be good stewards.”

Yet seven in 10 Americans believe global warming is occurring, according to a Yale University survey, and more than 60 percent say they are at least “somewhat worried” about its effects.