During the Democratic primary, one of the major climate policy differences between Bernie Sanders and Hillary Clinton was a carbon tax. He supported one. She didn’t. Now we have a little insight into why, via WikiLeaks’ recent release of thousands of hacked personal emails from Clinton campaign manager John Podesta.

“We have done extensive polling on a carbon tax,” Podesta apparently told Clinton adviser Jake Sullivan back in January 2015. “It all sucks.”

Some of that polling can be found in this leaked presentation by Anzalone Liszt Grove Research to the Clinton campaign dating March 2015. (Note that the campaign has avoided commenting on the veracity of any of the WikiLeaks emails.) The survey found that when people initially heard about a carbon tax, 58 percent supported it. But after hearing more detailed arguments for and against, support plunged to 46 percent:

You can see the pro and con arguments tested here. On the pro side, voters were told that the carbon tax would reduce fossil fuel pollution and that $1,500 per year worth of revenues would be kicked back to consumers. On the con side, voters heard some of the likely conservative arguments against, warning voters it would amount to “the biggest tax increase in history.”

It’s unclear whether the campaign commissioned other polls on carbon taxes besides this one, but officials seemed to conclude that the policy would be a liability. In June 2015, Politico’s Elana Schor wrote a piece quoting Sen. Chuck Schumer (D-NY) saying that Clinton could pursue a carbon tax if elected president. The campaign discussed internally how to respond to that story, as you can see in this hacked email thread.

Clinton campaign manager Robby Mook wrote that he didn’t want to explicitly rule out a carbon tax, since that might give Sanders room to run against them in the primary. But Mook also didn’t want to support the idea, because “it’s lethal in the general.” His full quote:

I don't recall any polling to guide us, but I'd be a bit nervous about rushing to say we'd never support such a tax. Bernie I assume DOES support such a tax and it could be fodder for him if we say unequivocally now that we don't support one. To be clear: it's lethal in the general, so I don't want to support one. But don't want to give bernie contrast right now. So if there's a way to re-state principles and say she'll announce something in the next few weeks, that would be great.

Publicly, the campaign ended up sidestepping the issue. In April 2016, Sanders pressed Clinton repeatedly at a primary debate, asking her whether she would support a tax on carbon. She demurred: “You know, I have laid out a set of actions that build on what President Obama was able to accomplish, building on the Clean Power Plan … I don't take a back seat to your legislation that you've introduced that you haven't been able to get passed. I want to do what we can do to actually make progress in dealing with the crisis. That's exactly what I have proposed.”

Even after activists succeeded in putting carbon pricing in the Democratic platform this summer, the Clinton campaign distanced itself from the language.

You can see a (hyper-cautious) logic at work here. By and large, Clinton’s climate and energy plan has focused on executive actions she can take as president without the help of Congress — it’s built around the assumption that Republicans in the House and Senate won’t cooperate anytime soon. So it seems the campaign didn’t want to go out on a limb for a policy that wasn’t likely to pass in the next two years and would cost them votes in the general election.

How much does this matter for climate change itself? In the short term, maybe not much. A carbon tax isn’t the be-all and end-all of climate policy, and there are plenty of other regulatory measures the next president can pursue to nudge down emissions piece by piece. Even in the long term, there are ways to decarbonize the economy without carbon taxes.

But economists tend to be quite insistent that carbon pricing would be a particularly efficient and elegant way to push the entire economy toward cleaner energy sources — and it’s hard to envision such a tax getting enacted anytime soon if presidential candidates are still afraid of even discussing it.

Other emails show Clinton aides grappling with the scale of the climate challenge

By the way, there’s another interesting thread in the WikiLeaks email dump that shows the Clinton campaign grappling with the sheer size of the climate challenge.

The emails are from June 2015, right after Martin O’Malley had just proposed zeroing out all fossil fuels by 2050. A Clinton aide, Josh Schwerin, asked others in the campaign if this was worth attacking at all: “I don't know much about the issue but zeroing out fossil fuels in 35 years seems unrealistic.”

Podesta pointed out that O’Malley was basically right about the scale of what’s needed to stop drastic global warming. “We need the get to 80% emission reduction by 2050,” he wrote. “Which implies close to a zero carbon energy sector.” (Note: This is correct.)

Jake Sullivan jumped in, a bit incredulous. “But he is saying zero fossil fuels, period, isn’t he? Transport, buildings, and energy? Or am I missing something?” He later added: “John — you think that is realistic?”

Podesta replied that getting to zero would be tough, particularly on transportation, but that’s the basic idea: “Hard to get all the oil out but wouldn't want to fight with him about that.”

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