President Barack Obama and Congress are headed for another power clash on the international stage, as key Senate Republicans challenge his efforts to forge a global pact on climate change.

The White House considers the agreement with nearly 200 nations a historic opportunity to reduce greenhouse-gas emissions world-wide. But some GOP senators view it as executive overreach, and they are quietly considering ways to warn other countries that the president doesn’t speak for them and may not be able to deliver on his promises to slash emissions.