America's liberal leaders are torn between fighting climate change and resisting nuclear power.

Why it matters now: The nuclear power industry, which provides the U.S. nearly two-thirds of its carbon-free electricity, is reaching an inflection point. Several power plants are shutting down under economic duress, which is putting pressure on Congress and state legislatures to keep them open, while a new generation of advanced nuclear technologies need government backing to get off the ground.

Some Democratic politicians and prominent scientists have come out to back nuclear in recent years because of climate change, but most of the biggest environmental groups and influential leaders remain opposed. In interview after interview at a United Nations climate conference in Bonn, Germany, I noticed a trend: Politicians would cite the many challenges facing nuclear power, such as safety, how to store radioactive waste and the economics, as reasons their positions didn't matter. Those more inclined to support the fuel would cite the challenges as hurdles to overcome. Three examples:

Tom Steyer , the billionaire environmental activist: "Obviously, nuclear does not create greenhouse gases. It creates other problems ... Nobody has any ability to create nuclear power at any kind of competitive price point, plus all of the existing nuclear plants have disposal and safety issues."

, the billionaire environmental activist: "Obviously, nuclear does not create greenhouse gases. It creates other problems ... Nobody has any ability to create nuclear power at any kind of competitive price point, plus all of the existing nuclear plants have disposal and safety issues." Michael Bloomberg , the billionaire former mayor of New York City, is opting not to put his money toward state initiatives trying to keep open struggling reactors. Instead, he's funneling his money — $116 million over the last month — to close coal plants in the U.S. and Europe. "Nuclear power is not killing people from air pollution and climate change the way coal power is," said Antha Williams, the head of the environment program at Bloomberg Philanthropies. "So he doesn't oppose nuclear."

, the billionaire former mayor of New York City, is opting not to put his money toward state initiatives trying to keep open struggling reactors. Instead, he's funneling his money — $116 million over the last month — to close coal plants in the U.S. and Europe. "Nuclear power is not killing people from air pollution and climate change the way coal power is," said Antha Williams, the head of the environment program at Bloomberg Philanthropies. "So he doesn't oppose nuclear." Democratic Gov. Kate Brown of Oregon, whose state is one of more than a dozen that effectively ban nuclear power, says she wants to see data ensuring safe storage of fuel waste. Brown described Oregon-based NuScale, a startup building small advanced reactor technologies, as "innovative," but she declined to comment on state legislation exempting the technology from the ban. "I would just say it's not my focus at this point in time."

Many of America's largest environmental groups, which have influence over liberal politicians, are doubling down on their opposition to nuclear power. They argue plummeting prices of wind and solar make nuclear power unnecessary.

Another reason: they'd lose donations, according to James Hansen, a climate scientist at Columbia University, and his colleague Steve Kirsch, a California-based entrepreneur and philanthropist. At a meeting in 2014 between Kirsch and Frances Beinecke, who at the time led the Natural Resources Defense Council, Beinecke said one of the reasons the group couldn't back nuclear power is because it would lose donations.

"The lunch did in fact occur and there was no movement," Kirsch said by email last week. A spokesman for NRDC declined to comment. Beinecke, who retired from NRDC later that year, didn't respond to requests for comment. NRDC's position on nuclear power resembles that of many others on the left: It would only support it if all of the industry's challenges are "properly mitigated."

Democratic senators who traveled to the Bonn conference indicated an increased albeit cautious openness to nuclear power, but this rhetoric was not matched by any sense of urgency to press for action in Congress or otherwise.

Sen. Sheldon Whitehouse of Rhode Island said there's bipartisan support to pass a pair of measures boosting advanced nuclear technologies and helping keep open existing reactors facing economic challenges. On the latter point, he was talking about a bill he authored that puts a price on carbon emissions. That would help nuclear power because it would monetize its carbon-free attribute, but Republicans, most of whom don't acknowledge climate change is a problem but do back nuclear power, don't support that bill.

"The rhetoric seldom goes further into doing anything that could actually support (or end) nuclear power in this country," said Andrew Holland, an energy expert at the American Security Project, a think tank. "What we're left with is a sort of stasis, where policies don't change, and America's nuclear power capacity slowly erodes."

Meanwhile, smaller policies seem poised to pass. The tax overhaul bill the House just approved extends a production tax credit for nuclear energy, which industry executives say is critical to both existing reactors and advanced technologies still in planning phases.

"It is shifting in a sensible direction, but slower than it needs to," Hansen said.

The Harder Line will be off next week and back Dec. 4.