Still, she was bothered by the questioning of his motives and the occasional charges of hypocrisy from the press and his political enemies. Yes, she patiently explained to me, Steyer’s firm made many millions from investments in coal mines and coal-fired power plants in Asia and Australia, as it had in all sorts of other industries. But that was long before he came to believe that the fate of the planet depended upon curbing carbon emissions, and he has since divested his personal portfolio of any fossil-fuel-related investments. Taylor had also learned to deflect the inevitable questions about whether he was preparing his own run for office. (“I don’t think he thinks that’s the right priority right now,” she said.)

Steyer has been involved in politics for years. He was a major fund-raiser for John Kerry’s 2004 presidential campaign and in 2006 worked closely with Chris Lehane, a longtime Democratic strategist, to defeat a Republican effort to change California’s electoral-college rules. (Lehane has been acting as Steyer’s personal political strategist ever since.) In 2012, though, Steyer read an essay in Rolling Stone (“Global Warming’s Terrifying New Math”), in which Bill McKibben, the writer and climate activist, suggested that fossil-fuel interests had too much money at stake to let the political system do anything about carbon emissions. The only alternative, McKibben wrote, was a mass movement, and the only way to start a mass movement was to articulate a consistent, fact-based moral argument for change. At that moment, Taylor said, “Tom realized that the climate threat was near, present, imminent, massive — and aggravates every other crisis, whether it’s hunger or civil rights.” He had to do something different. So a few months later he and Taylor invited McKibben to the ranch for a war council.

We were in the ranch’s spartan barn house when Taylor recounted this. The council had been right there, beneath the reclaimed wood beams. Lehane was there, and so was John Podesta, Bill Clinton’s former chief of staff who would later join the Obama White House as an adviser. They all considered various ways to act. They agreed, finally, that Steyer should try a few experiments, but should come out with particular force against the Keystone XL pipeline, which would link oil-rich Canadian sands to refineries on the Gulf of Mexico. In March 2013, Steyer sent a letter to Stephen Lynch, a Democratic congressman and pipeline supporter who was seeking the Senate seat left vacant by John Kerry when he became secretary of state. The letter demanded that Lynch renounce his support for Keystone by “high noon Friday” or face aggressive opposition. It was an obvious stunt, but it got attention. Lynch declared that “billionaires won’t shove me around”; Steyer responded by providing Lynch’s primary opponent, Ed Markey, an anti-Keystone congressman, with nearly $700,000 in campaign support. Markey, who said he did not want Steyer’s help, won handily. In Virginia that same year, Steyer bought $8 million in television ads and mailings to highlight the climate-change skepticism of the Republican gubernatorial nominee Ken Cuccinelli. The Democrat, Terry McAuliffe, a former chairman of the Democratic Party, won by 2.5 percentage points.

How much credit Steyer can claim in either of these victories is debatable. But by early 2014, just about every Democrat considering a competitive run this year, including Charlie Crist, had made a pilgrimage to see Steyer in San Francisco. And in April, the State Department announced it would delay its decision on Keystone XL pending an analysis of expected legal challenges.

It was a victory, but it did not necessarily echo the consistent, moral argument of McKibben’s initial call. As we spoke, NextGen was spending about $100,000 to build and operate a wooden Noah’s Ark on wheels that it would drive around the state, taunting Scott for his failure to address climate change. It was also spending $2 million on an ad campaign that blamed Scott for the failure of Duke Energy, a major Scott contributor, to upgrade one nuclear power plant and build another, for which it had to increase utility fees, despite the fact that Scott had no direct say over this decision. Even beyond the publicity stunts and elisions of fact, NextGen’s campaign largess was itself a capitulation to the post-Citizens United world. Steyer was applying pressure to the political system in a way no average American could. It seemed undemocratic. But Steyer saw it as simple pragmatism; the other side was “playing multiples,” he said, and he had to operate “in the real world the way the real world works.”

Our tour wound down in front of a long, waist-high granite pool filled with koi, the remains of an unsuccessful attempt at sustainable fish farming that now doubled as a huge outdoor dining table. As we stood beside it, Taylor told me about charts she had seen, apparently online, placing Steyer at the center of a web of liberal interests: a conservative answer to the many liberal diagrams of “The Kochtopus.” The connections were all real, or at least many of them were. That was the whole point of playing multiples. But the comparison seemed to bother her. “There’s a private empire and the Earth,” she said. The Earth was there for all of us.

“And the private empire is?” I asked.

“Private.”

And it was true. But it was also undeniable that Steyer presided over an empire of his own that, for all his transparency, was not quite public.