President Joe Biden on Monday sought to assure world leaders the United States would fulfill its promise to slash greenhouse gas emissions in half by the end of the decade, but a setback at home heightened uncertainty about his ability to follow through.

Biden joined leaders from over 100 countries in Glasgow for the start of the COP26 climate conference, which kicked off on the heels of the G20 summit in Rome that concluded with a statement that urged "meaningful and effective" action on climate change but...