There isn’t much daylight these days between the Democratic candidates on the environment. Bernie Sanders, Hillary Clinton, and Martin O’Malley all agree that humans are responsible for climate change and that it’s one of the world’s most pressing problems. To that end, they support clean energy tax breaks, reject drilling offshore and in the Arctic, and oppose the (now-rejected) Keystone XL pipeline.

But there’s one environmental issue where Sanders truly stands apart: He wants to ban hydraulic fracturing outright. Clinton and O’Malley have proposed lesser measures, and show no sign of going further. That’s an indication of just how radical Sanders’s stance really is, but it also raises an important question: Is a fracking ban remotely plausible?

There used to be more daylight between the candidates, especially Sanders and Clinton. The Vermont senator has long called for “a political revolution that takes on the fossil fuel billionaires, accelerates our transition to clean energy, and finally puts people before the profits of polluters”—and he’s taken early, decisive stances in support of many of the environmental movement’s top demands before he ever launched his presidential campaign.

Clinton has followed Sanders’s lead to the left. She came out against the Keystone XL pipeline just months before President Barack Obama rejected it. She knocked the dangers of Arctic drilling last August, as Shell faced increasing scrutiny and abandoned its exploratory drilling just one month later. And she opposed Obama’s Trans-Pacific Partnership trade deal on the grounds it would hurt American jobs and wages, but the move earned her points with green groups who opposed the deal for other reasons. That’s not an exhaustive list of every issue environmentalists care about, but it was enough to earn the Clinton campaign an endorsement from the League of Conservation Voters.

Sanders took these very same positions long before the launch of his campaign, a green radicalism that Clinton has somewhat diluted. But a fracking ban remains safely his issue. Whether that’s an advantage or disadvantage is another question entirely.