The environmental policy debate between Democratic contenders Bernie Sanders and Hillary Clinton could soon become much more heated.

Leading up to Super Tuesday, a slew of contests that could make or break Bernie Sanders's candidacy, the Vermont senator has gone on the offensive on the surprising issue of fracking-the controversial method of drilling for natural gas. The campaign last week made several media buys in Minnesota and Colorado for a new ad that touts Sanders as the only candidate who firmly opposes fracking.

The fracking ads look like a last-ditch effort by Sanders, who was badly beaten by Clinton in the recent South Carolina primary, to differentiate himself from the former secretary of state. The ads tout Sanders as "the only candidate to oppose fracking."

Fracking is the process of drilling into shale formations and injecting a cocktail of water, sand, and chemicals to create tiny fractures that access pockets of oil and natural gas. The process has helped fuel a natural gas extraction boom in the United States and made the nation the largest natural gas producer in the world. But it has also become a flashpoint for environmental outrage, due to research that links fracking with contamination of local water and air, and even with abnormal earthquakes.

Sanders's fracking ads appear designed peel away liberal support from Clinton, particularly in such states as Colorado and Minnesota, where fracking is hotly debated and which experts have said are must-wins for Sanders.

In the ads, Sanders has set out to link his constant attacks on Clinton for accepting big money from special interests to the hot-button issues of fracking and climate change. The ads proclaim that Sanders has "the courage to stand up to big oil, because he won't take their money"-an indirect swipe at Clinton.

Sanders has attacked Clinton for taking money from the oil and gas industry, pointing specifically to a fundraiser she attended at Franklin Square Capital, a hedge fund that invests in fracking. A couple weeks later, Clinton released a policy report detailing her support for continued natural gas production.

"Just as I believe you can't take on Wall Street while taking their money, I don't believe you can take on climate change effectively while taking money from those who would profit off the destruction of the planet," Sanders said at a campaign rally in Hibbing, Minnesota, last week.

Sanders has already succeeded in pulling Clinton to the left in the primary on another environmental issue, namely the Keystone XL pipeline. Both Sanders and former contender Martin O'Malley had criticized Clinton for not articulating a position on the issue before she eventually came out in opposition. And while she has joined with Sanders to support a ban on fracking on federal lands, Clinton has come under scrutiny in the past for setting up a State Department program aimed at expanding fracking overseas.

"Under her leadership, the State Department worked closely with energy companies to spread fracking around the globe," Mother Jones reported in 2014, "part of a broader push to fight climate change, boost global energy supply, and undercut the power of adversaries such as Russia that use their energy resources as a cudgel."

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Some environmentalists and energy experts argue that natural gas, which emits less carbon than other fossil fuels like coal, can serve as an "energy bridge" from fossil fuels to clean renewables. Clinton has come to embrace that position, though she advocates for strict federal regulation.

Influential environmental groups, however, have denounced Clinton on the issue. The risk for methane pollution, which can leak during the drilling process, negates any decrease in carbon emissions, argue the Environmental Defense Fund and other fracking opponents. Fracking opponents also argue that evidence of water and air contamination is too great to ignore.

Sanders echoes that sentiment, rejecting fracking and instead proposing a clean energy plan that pushes for a much more rapid transition to clean energy. He's called for closing the so-called "Halliburton loophole," a policy that exempts the fracking industry from adhering to the Clean Water Act, which sanctions water polluters. A Sanders administration, as specified in a recent Grist article, would be bullish on using executive power to rein in the fracking industry.

Clinton, for her part, maintains that natural gas is an important part of the country's clean energy future, but that it has to be done carefully. "We have to face head-on the legitimate, pressing environmental concerns about some new extraction practices and their impacts on local water, soil, and air supplies," Clinton said at the National Clean Energy Summit in 2014, specifically mentioning methane leaks. "So it's crucial that we put in place smart regulations and enforce them, including deciding not to drill when the risks are too high."

It's an open question whether Sanders can draw environmental voters away from Clinton. At least one leading environmental group-the League of Conservation Voters Action Fund-endorsed Clinton last fall, and has been deploying volunteers on her behalf. However, most major green groups have not made an endorsement.