From the start of her 2016 campaign, Hillary Clinton has worked methodically to prove she is a candidate the environmental movement can trust—a process that will no doubt continue in the third Democratic debate on Saturday night. She’s come out against the Keystone XL pipeline and Arctic drilling, and just this week said she’s skeptical of offshore drilling. She frames climate change as “a real threat to our planet.” She’s rolled out policies that back up her claims that she’d expand clean energy and reduce fossil-fuel reliance as president. And her campaign chair, John Podesta, told me in October that Clinton intends to make climate change a key issue of differentiation from the Republican nominee in the general election, assuming she’s the nominee. 

Despite all this, Clinton hasn’t quite been able to allay doubts about her commitment to climate and environmental priorities. Although she did secure an endorsement from major green group League of Conservation Voters Action Fund this fall, many environmentalists remain skeptical. LCV’s endorsement provoked such a backlash that the group later felt compelled to defend its decision, insisting, “she will be the most effective leader to fight the big polluters and win on climate.” Activists with Action continue to press Clinton at her campaign stops about her weak spots, including one Iowa activist who asked her this week about her ties to the fossil fuel industry. There are many people in the climate community that understand Hillary is only moving slowly on climate, and only when pushed,” R.L. Miller, a climate activist with the group Climate Hawks Vote, told ThinkProgress last month

Why are they nervous? It mainly comes down to Clinton’s history and connections. In the past, she supported some of the same policies she’s now against. As secretary of state, she said she was “inclined” to approve the Keystone XL pipeline permit. As a senator, she once took a different view of offshore drilling, as well, voting with Republicans (pre-BP disaster) to pass a bill that opened up 8 million acres in the Gulf of Mexico to offshore drilling.

On top of that, Clinton has stronger ties to the fossil-fuel industry than her Democratic counterpartsnot to mention many Republicans. In 2014, she campaigned for Keystone XL pipeline supporter Mary Landrieu, who was fighting to keep her Senate seat in Louisiana. (Landrieu has returned the favor.) Clinton has also taken at least $150,000 in oil and gas contributions, which Huffington Post notes is more than any of the GOP presidential candidates. 

Clinton maintains she wasn’t aware of those donations. “They certainly haven’t made that much of an impression on me if I don’t even know it,” she told at an Iowa town hall. But for many in the green movement, the oil and gas money only reinforces the suspicion that the industry has some sway over Clinton’s climate policies.

It doesn’t help, either, that she hasn’t pledged to ban fossil fuel extraction on federal lands—environmentalists’ top priority now, after Keystone. She struck a middle ground on permitting coal, oil, and gas production to continue, much like Obama has. Sanders, on the other hand, has called for halting future production on public lands.

Clinton’s political motivation for adopting a more aggressive climate agenda is clear: President Barack Obama has made climate change an unavoidable issue in the 2016 election, and the Clinton campaign is counting on Republicans’ unique form of climate denial to boost her chances to win the presidency. But first, she has to secure the nomination, and to maintain her edge over Bernie Sanders—who has a long environmental record in the Senate and boasts backers like environmentalist Bill McKibben. 

If Middle East politics doesn’t completely dominate the debate on Saturday night, as it did in the fifth Republican debate, then the Paris climate agreement will be a topic of discussion. It is much more likely to come up for Democrats than for Republicans because Clinton, Sanders, and Martin O’Malley want to talk about it. Clinton and O’Malley have embraced the historic accord, while Sanders has been less supportive, saying the U.S. needs to do much more. The question for all of the candidates is much the same: How far are they willing to go to build on the Paris momentum?

Clinton won’t allay doubts about her commitment to climate action by restating her positions—even the ones the environmental movement supports. She could do more, and she might, especially if Sanders makes another surge before the caucuses and primaries begin. Clinton still has work left to do to convince environmentalists that her support for climate change action is more a matter of principle than politics.