Billionaire climate activist Tom Steyer called President Trump a traitor if he makes good on his promise and exits the Paris climate change agreement.

"If Trump pulls the US out of the #ParisAgreement he will be committing a traitorous act of war against the American people," Steyer tweeted.

Steyer was very active during the presidential campaign through his group, NextGen Climate, and has become a leading activist in an agenda that supports moving the nation toward 100 percent renewable energy over the next 25 years.

He joins a number of environmental groups criticizing Trump's decision to withdraw from the Paris Agreement.

It's an "epic blunder," said David Arkush, managing director of Public Citizen's climate program, the group founded by former Green Party candidate Ralph Nader.

Erich Pica, president of Friends of the earth, called on all nationsto impose sanctions on the United States in response to Trump's decision.

All nations should "assert heavy economic and diplomatic pressure to compel the Trump administration to take serious climate action to protect people and the planet," Pica said.

The Environmental Working Group said Trump joins the ranks of Syrian dictator Bashar Assad in rejecting the Paris climate deal. Syria and Nicaragua are the only two countries that did not sign onto the 197-nation accord.

"Pulling the nation out of this pact would only further cement in the minds of Americans and others around the globe that the president has no sense of moral responsibility to protect future generations from the ravages of climate change," said the group's president, Ken Cook.