For the majority of environmental funders and others actively involved in fighting climate change, CNN’s recent “Climate Crisis Town Hall” must have seemed like a windfall, as 10 leading Democratic candidates waged what one analyst for the network called “a bidding war to show liberal activists their plan was the most audacious—and even expensive.” But for a small but dedicated corner of climate philanthropy, it was an unmitigated disaster, as one of the surprisings “bids” was a blanket pledge by Senator Elizabeth Warren to rid the United States of nuclear energy by 2035. According to one such funder, Warren’s vow came as a “gut punch,” as the Massachusetts senator is widely considered a serious student of policy willing to explain and sell complicated tradeoffs to voters. And it followed on the heels of another anti-nuclear blow delivered via cable television,HBO’s runaway hit series Chernobyl, which gruesomely—and, according to some of these experts, inaccurately—portrays the most feared downside of nuclear power. Despite these setbacks, more than a dozen funders, grantees and others in the nonprofit pro-nuclear space recently interviewed by Inside Philanthropy say they remain...