Not a single Republican mayor attended a two-day climate change summit at the Vatican this week. While dozens of U.S. mayors of both parties were allegedly invited, only Democrats accepted, U.S. News has learned.

The one-sided turnout may partly be due to the hastily assembled guest list – Republican mayors James Brainard of Carmel, Indiana, and R. Rex Parris of Lancaster, California, who have spearheaded efforts to address global warming, say they were not invited. But the RSVPs also suggest how deep the political divide over global warming now goes, spurring even local GOP mayors to spurn a prestigious invitation from the papacy.

"Republican voters, the people out there, they've actually turned the corner in actually recognizing that climate disruption is caused by pollution. But the leadership, all the funding comes from the people who don't want people to believe that," says Parris, a self-described "right-wing conservative" who harnessed solar power to slash emissions in his desert city of 160,000.

Mayors and other guests listen to Pope Francis' speech on climate change and slavery L'Ossservatore Romano/AP

Most GOP voters say they want politicians to address global warming, but conservative action groups – like those backed by coal, oil and natural gas groups, as well as the billionaire industrialists Charles and David Koch – have not only undermined climate science but made examples of the politicians who support acting on it. That presents a strong incentive for mayors with state or national designs to simply stay home.

"The fear is there in the people who want to go on to higher office," Parris says.

A spokesman for the Vatican summit declined to state which mayors refused the invitation, which were allegedly sent to larger cities by the Pontifical Academy of Sciences, a Vatican-funded research group. Of the cities that have Republican mayors – including Albuquerque, Indianapolis and San Diego – officials say they either were not invited, "had no record" or "have no knowledge" of having received an invitation.

About 60 mayors from around the world attended the summit, including 10 Democrats from the U.S., among them New York City's Bill de Blasio and San Francisco's Edwin Lee. On Wednesday, they signed a declaration that "human-induced climate change is a scientific reality, and its effective control is a moral imperative for humanity."

New York Mayor Bill de Blasio held a press conference the first day of the summit Tuesday. Gregorio Borgia/AP

The statement came on the heels of Pope Francis' encyclical on climate change last month, a formal letter on Church doctrine that was the first ever to focus exclusively on the environment. During remarks to the mayors Tuesday, he called cities a linchpin in halting global warming and other issues, including human trafficking and inequality.

"You are the conscience of humanity," Francis said.

About half the world's population lives in cities, which together account for three quarters of human emissions. Changes to their design – combined with local political action – can help achieve the carbon reductions that are often far harder to accomplish on the national and international level, says Brainard of Carmel (population: 86,000), who has long advocated cities' role in reducing global warming.