Today, Democrats go to the polls in ten states. With 865 pledged delegates at stake in the Super Tuesday primaries and caucuses — along with an additional 150 superdelegates — today’s contests will determine about 23 percent of the total delegates to the Democratic National Convention in July, where the party’s nominee will ultimately be decided.

Hillary Clinton leads in almost every state except Massachusetts, Colorado, and Vermont, enjoying especially high support in southern states like Georgia and Alabama. The other exception is Oklahoma, where Clinton and Bernie Sanders are virtually tied. Two weeks ago, polls reported that Sanders was projected to receive 44 percent of the vote, just a couple points behind Clinton’s 46 percent — well within the margin of error. Polls released yesterday show a five-point lead for Sanders — 48 percent to Clinton’s 43, with 8 percent undecided — but this difference also falls within the margin of error.

Politically, there’s much to distinguish the two presidential contenders. Clinton represents the conservative mainstream of the Democratic Party, while Sanders’s campaign has been likened to an insurrection within party ranks. While Sanders proposes ambitious reforms like single-payer health care and free higher education, Clinton promises a continuation of Obama administration policies, emphasizing her political savvy and experience as secretary of state.

But one issue in particular may play a central role in determining how people vote in today’s contests: hydraulic fracturing, or fracking — a controversial oil and natural gas extraction method that ruins drinking water, causes earthquakes, and releases potent methane gas into the atmosphere. Fracking accounts for 43 percent of US oil production and produces 67 percent of the country’s natural gas, generating billions of dollars in revenue for American energy companies each year.

Sanders is the only presidential candidate who is opposed to fracking. His platform calls for a nationwide moratorium modelled on the fracking ban already in effect in his home state of Vermont. Clinton, on the other hand, has accepted billions from the same natural gas companies that profit from this environmentally devastating process. And, as secretary of state, she lobbied other world governments to allow fracking within their borders, clearing the path for American firms such as Chevron to sign lucrative overseas contracts.

Like the GOP presidential contenders, Clinton wants to expand fracking. Sanders wants to abolish it.

In the build-up to today’s contests, Sanders mounted a late-in-the-game offensive in three states — Oklahoma, Colorado, and Minnesota. Each of these states has been severely affected by the fracking industry in recent years, a reality not lost on the democratic socialist candidate.

The Sanders campaign allocated a portion of its tightly rationed television budget to run anti-fracking ad s in Colorado and Minnesota. And while visiting Oklahoma on Friday, the Sanders campaign released a statement putting his position on fracking in the starkest terms possible. After noting a recent Clinton fundraiser organized by a group of fracking investors, the statement included the following quote from Sanders: “I do not support fracking. I don’t need money from hedge fund managers and I don’t want money from those who profit off of the destruction of our planet.”

Sanders’s principled opposition to fracking resonates with his broader critique of inequality in American society. Fracking enriches energy companies and their investors while jeopardizing the wellbeing of ordinary people, as Sanders pointed out in a statement referencing flammable water and the wave of fracking-related earthquakes in Oklahoma.

There were 907 earthquakes above a three-point magnitude in Oklahoma last year, up from only two before fracking activity began in earnest in 2009. And the earthquakes show no signs of stopping or slowing down — there have been more than three hundred in the state since the start of this year, averaging more than five a day. Several studies have linked the wave of earthquakes to fracking, suggesting that the injection of wastewater fluids into underground disposal wells has the effect of lubricating subterranean faults, which, in turn, produces increased seismic activity.

Oklahomans have come forward to express their outrage, voicing concerns about property damage and compromised safety. Class action lawsuits filed last month in several Oklahoma counties take aim at energy companies, reporting structural damage to homes and alleging that wastewater injection constitutes an “ultrahazardous activity.”

In Colorado — where recent polls show Sanders with a slight lead — citizen advocates have successfully pushed for some of the strongest industrial disclosure laws in the country to combat irresponsible frackers. Colorado currently has more than 45,000 drill sites, despite growing popular opposition to their installation. In 2013, widespread flooding damaged pipelines and distributed fracking pollutants all over the state.

In Minnesota, residents have raised concerns over silica mining, which releases carcinogens and demolishes entire hillsides of quartz with explosives and other highly invasive mining techniques. Colloquially known as “frac sand,” the coarse sediment is combined with a chemical cocktail which is then injected into underground shale, fracturing it and releasing the natural gas underneath. Minnesota’s mines satisfy the massive demand for frac sand in nearby North Dakota and other energy-rich regions across the country.

Hydraulic fracturing has economic as well as ecological effects. The industry is associated with a boom-and-bust production cycle, in which millions of dollars in new investments are poured into gas-rich regions only to be abruptly extracted when the profits dry up.

People once flocked to fracking hubs in North Dakota seeking employment; now workers increasingly find themselves laid off and destitute, with some even sleeping in their trucks or in utility sheds. In western Pennsylvania, a recent downturn in fracking activity has caused a spike in home foreclosures in several of the state’s impoverished rural counties.

More troubling still, the predatory land speculation that goes hand in hand with fracking has been responsible for the demolition of trailer parks and the displacement of their residents. Despite legal protections for locals who lease portions of their property to gas drillers, many cash-poor landowners have been cheated out of the gas royalties to which they’re entitled.

Fracking generates billions of dollars in profits each year, but almost none of that makes it back to the communities where the oil or natural gas originated. Instead, energy companies get richer while local residents are left to deal with the consequences — including economic downturns and polluted water sources.

Sanders has drawn a line in the sand in battleground states like Oklahoma and Colorado, sharply distinguishing himself from his fracking-friendly opponents. And this position could have effects beyond Super Tuesday, as his message may resonate with primary voters in other key states — like Ohio, Pennsylvania, and West Virginia — who have witnessed the ecological and economic degradation caused by hydraulic fracturing.

Over the course of this election season, the Sanders campaign’s attention to environmental issues has forced Clinton to tack left on several issues — including the Keystone XL pipeline and Arctic drilling — in an effort to maintain her hold on environmentally concerned members of the Democratic base.

But Clinton’s connections to oil and gas interests are too strong for her to drift even more to the left on this issue — she has gone no further than to suggest lukewarm reforms like raising gas extraction rates on public land and introducing automatic safety valves to help prevent fracking-related methane leaks. Ultimately, this is where Clinton’s liberal environmentalism breaks down.

For Clinton, the environmental crisis is a special-interest issue, best solved gradually through technological innovation and clever policy changes aimed at encouraging increased investment in clean energy. But Sanders’s wholesale opposition to fracking demonstrates a robust environmentalism, connected to his alternative economic vision.

Sanders recognizes the problem for what it is, getting to its root by pointing out the disproportionate wealth and power of energy companies relative to the communities they affect. Environmentally degrading practices like fracking generate tremendous wealth for some while contributing to the economic precarity of many, threatening ordinary people’s access to necessities like water and forcing the public to foot the bill for costly clean-ups and restorative programs.

Fracking has no place in the more equitable economy Sanders and his supporters envision. And today, as Democrats go to the polls, voters in Oklahoma, Minnesota, and Colorado may affirm that fracking has no place in the future they envision either.