PORTLAND — Maine's two U.S. senators want their colleagues to ratify an agreement that would phase out a greenhouse gas linked to climate change.

Republican Sen. Susan Collins of Maine and Democratic Sen. Chris Coons of Delaware sent a letter in late April to the chair and ranking member of the Senate Foreign Relations Committee calling for ratification of the Kigali Amendment to the Montreal Protocol. Independent Sen. Angus King of Maine was among a bipartisan group of senators to co-sign the letter.

The amendment is designed to phase down hydrofluorocarbons. The senators said ratification of the amendment would help the U.S. in the international marketplace because American companies have already led the way in transitioning to substitutes.

“U.S. industry has for years positioned itself as a leader in the effort to develop alternatives to hydroflourocarbons, including for use in heating, ventilation, air conditioning, and refrigeration,” the senators wrote.

Dozens of countries have already ratified the Kigali Amendment.