Hillary Clinton’s campaign is leaving the door open to supporting a carbon tax, hinting that the Democratic nominee could eventually back the controversial idea. Statements from top campaign officials made during the Democratic National Convention in Philadelphia could endear her to environmental activists who are pressing her to adopt more of the progressive positions of primary rival Bernie Sanders — a vocal carbon tax supporter. But it could be a difficult decision for Clinton with clear political costs. Putting a price on carbon dioxide emissions — which would raise the costs of fossil fuels — has rarely gone over well politically, and could open Clinton up to attacks from Donald Trump and conservatives.  At a League of Conservation Voters event in Philadelphia, Clinton campaign chairman John Podesta toldPolitico that “if Congress wants to come forward with [a carbon tax proposal], we’ll take a look at it.” Trevor Houser, Clinton’s top energy adviser, had a similar take at a separate event, saying that “if Congress wants to have a conversation about addressing climate change, Secretary Clinton would be delighted to have that conversation.” Both men were careful to add caveats to their answers, making it clear that Clinton is not proposing a carbon tax, and her chief climate plan is to build upon President Obama’s agenda through regulation. “Democrats believe that climate change is too important to wait for climate deniers in Congress to start listening to science,” Houser said. The statements, though, nonetheless move Clinton past her previous refusal to even talk about the issue. During the primary fight with Sanders, she tiptoed around the issue. Sanders repeatedly pushed Clinton to take a position on carbon taxes and said that her silence meant she did not take the climate issue seriously enough. Sen. Chuck Schumer (D-N.Y.), who’s on track to lead the Senate Democratic caucus in 2017, said last year that a carbon tax is likely to pass the Senate if Clinton wins and Democrats retake control of the upper chamber. Clinton supported cap-and-trade, another mechanism for putting a price on carbon emissions, in the 2008 campaign, before legislation for such a plan died in the Senate in 2010. Now she’s walking a fine line on carbon pricing, keeping the door open, but not too enthusiastically. Barry Rabe, a public policy professor at the University of Michigan, said her refusal to rule out a carbon tax is telling. “What this does is further suggest that it’s possible, post-election, that some form of carbon price could be on the table. We’re not hearing the campaign saying ‘absolutely not,’” Rabe said. He added that a few factors could push Clinton more toward a carbon tax, either during the campaign or if she wins the election. Chief among them would be the need to win over progressive Sanders supporters. “I can’t help but think she’ll be looking for ways to do that, not just on climate,” he said. But she also might believe that regulation isn’t enough to tackle the scope of the climate challenge. Carbon pricing has wide support among liberals and among some conservative economists, as a relatively straightforward way to reduce carbon emissions, depending on how it is structured. The Democratic Party platform this year also endorses a price on carbon. But Republican lawmakers and Trump are steadfastly against it. Most of the party’s leaders, including Trump, doubt the impact of greenhouse gases on climate change, and see a carbon tax as little more than an increase in energy costs. Republicans passed a symbolic resolution in June denouncing a carbon tax, hoping to close the door completely on the question. Clinton’s delicate handling of the issue is probably informed in part by the political history of energy taxes. The Democratic-led House passed a “BTU tax” — for British thermal unit, a measure of energy — in 1993, costing the party its majority the next year. After Democrats retook the House, they passed a cap-and-trade bill in 2009, and again lost the majority in 2010. “Clinton has no intention of being suckered into a political disaster by advocating a carbon tax,” said Paul Bledsoe, a political consultant who worked for the Senate Finance Committee’s Democrats when the House passed its tax in 1993. He later served as the spokesman for the White House’s Climate Change Task Force under President Bill Clinton. “If Republicans will come out for it and vote for it, that’s a different matter. But until that happens, the Democrats should have nothing to do with it, because it’s political poison,” Bledsoe said. Rep. Kevin Cramer (R-N.D.), an adviser to the Trump campaign, said any change from Clinton is likely to be a liability for her. Trump has already criticized her for endorsing an expansion of Obama’s environmental policies, and a carbon tax would make that attack easier. “Any time a candidate for any office — especially president — leaves the door open for any type of a new tax, you’re making yourself somewhat vulnerable, politically,” Cramer said. The Trump campaign didn’t respond to the statements from Podesta or Houser. Conservative anti-tax advocate Grover Norquist, who leads Americans for Tax Reform, said the Democrats probably lost the 2016 election already by including a carbon tax in their platform. “When counting to 270 — the number of electoral votes needed to win the presidency — the Republicans may have already won the election in five short words: ‘We oppose any carbon tax,’” he said in a statement. Norquist pointed particularly to Pennsylvania, Ohio and Colorado as swing states, crucial to the election, but also home to large increases in oil and natural gas drilling in recent years that could be slowed down by restrictions on carbon. Cramer too said Republicans would benefit from even having Clinton openly consider a carbon tax. “It all adds up to an almost unarguable pattern that she wants to continue Obama’s policies,” he said. “It makes it easier than ever to tie her to his agenda. And I think that’s a sweet spot for our team.”