"We are showing up in force," Gina McCarthy, co-chair of the "America Is All In" coalition told reporters on a call Thursday.

The group represents around "two-thirds of the US population and three quarters of the US GDP, and more than 50 percent of US emissions," said McCarthy, who served as a climate advisor to former president Joe Biden, and as ex-president Barack Obama's environment chief.

President Donald Trump announced he was withdrawing the United States from the Paris climate accord for a second time on his return to office in January.

But McCarthy said that would not halt American participation in global climate efforts.

"We'll deliver on the promises we made to the American people and our international colleagues," she said. "Local leaders here have authority to act on their own behalf, to take climate action at home and abroad."

She pointed to the work of the 24-state "US Climate Alliance" that have slashed emissions by a quarter relative to 2005 while growing their economies.

Because the Paris accord requires a one-year notice period for withdrawal, the United States remains a party for a few more months.

But Democratic Senator Sheldon Whitehouse, who was also on the call, said it appeared unlikely the administration would send an official delegation to COP, given it had not put in embassy support for the Americans attending.

"But who knows?" added Whitehouse. "This is a very mercurial administration. They can decide at the last minute to send a plane to Belem, full of climate deniers and fossil fuel operatives."

While Trump also exited the Paris deal in his first term, his administration has gone further this time, exerting its clout to boost fossil fuels globally.

This includes, for example, threatening countries with retaliatory measures if they agreed to a carbon pricing system by the UN's International Maritime Organization, effectively curtailing its implementation.

Climate advocates fear the administration could seek to withdraw from the UN Framework Convention on Climate Change -- the treaty that underpins the Paris Agreement.

Doing so could prevent future administrations from re-entering the deal, but it is not clear if the executive branch has the legal authority to undo a Senate-ratified treaty.