Trump's goal appears to be to eventually unwind the Clean Power Plan, which Obama put in place in 2014 to require states to reduce carbon dioxide emissions by about one-third of 2005 levels over the next 15 years.

Republican lawmakers HATE this plan. It's filled with regulations they worry will hurt energy companies in their state, like coal workers in Senate Majority Leader Mitch McConnell's Kentucky.

But environmentalists saw the Clean Power Plan as the best way to reduce emissions in the absence of a Congress reluctant to. The political tension pretty much guarantees that as Trump rolls back greenhouse gas emission regulations, environmental groups will sue him for it. (In April, a federal court granted the Trump administration a 60-day pause on all lawsuits about the Clean Power Plan while the Trump administration reevaluates it.)

These Republican senators argue that if Trump is still in the Paris deal, environmental groups can hang their lawsuit on it. “It is clear that those advocating for greenhouse gas regulations will use the Paris Agreement as a legal defense again,” the letter reads.

What the other side says: Trump's going to get sued anyway. “Tearing the rules down require going through the same process it took to build them up,” David Doniger, director of the Natural Resources Defense Council’s climate and clean-air program, told The Washington Post in March. “We will make them face the music at every step.”

Argument three: If you stay in the agreement, China will win

The United States is one of the world's largest greenhouse-gas emitters, and its participation was expected to make up 21 percent of the emissions the Paris deal would cut, says The Post's Chris Mooney.

By comparison, China gets off easy under the Paris agreement, GOP senators argue. Barrasso in the Washington Times:

“The Paris deal imposed on the United States unrealistic targets for reducing our carbon emissions. It set America’s standards higher than for much of the world, while giving countries like China a free pass for years to come.”

What the other side says: Since signing the Paris agreement, China has stepped up its game on reducing carbon emissions, largely because its pollution is so bad it has no choice. China's appetite for coal is declining, and “both China and India look set to overachieve their Paris Agreement climate pledges,” predicted the Climate Action Tracker in a May report. If China follows through on its intent to lower emissions more than it promised in Paris, it could even cancel out any increased emissions from the United States.