Left-wing climate activists are stepping up their opposition to policies from top Democrats promoting nuclear energy and carbon capture, signaling a tough debate as the Biden administration advances legislation to curb power sector emissions.

Hundreds of environmental and grassroots groups, including national groups such as Friends of the Earth, Center for Biological Diversity, and the Indigenous Environmental Network, sent a letter to lawmakers Wednesday calling on them to pursue a national standard requiring 100% renewable power by 2030.