Fossil fuel interests have pumped $3.25 million into the largest super PAC supporting Hillary Clinton's candidacy for the White House.

Approximately one in every 15 dollars given to Priorities USA Action, which took in $50.5 million in contributions last year, came from donors linked to oil and natural gas interests, according to data compiled by Greenpeace.

But fossil fuel interests are also sending checks directly to her campaign. Clinton has take in nearly $268,000 in contributions from individuals employed in the oil and gas sector this election cycle, according to the Center for Responsive Politics. Her rival, Vermont Sen. Bernie Sanders, who does not have a super PAC spending on his behalf, took in just over $35,000.

By comparison, direct contributions from employees in the pharmaceutical and health products industry have reached approximately $410,000 for Clinton and $82,000 for Sanders so far this election cycle.

Greenpeace, which does not endorse candidates for political office, said the contributions highlight the role of special interests in politics, which stands in the way of "sensible" climate and environmental policy.

"When people walk into a room with $3.25 million, in a political system where you have to raise this kind of money to win, you're going to have to make concessions," said Jesse Coleman, a researcher for Greenpeace. "That's the danger right there, that you're listening to them more than you're listening to people who don't have that kind of money."

Bill McKibben of 350.org, who has endorsed Sanders, said the fossil fuel industry "should be persona non grata for any politician who cares about climate change."

"They've lied and lobbied for decades to keep meaningful action at bay — all of us, almost as a matter of intellectual hygiene, should do everything we can to keep them at a distance," he said.

Priorities USA Action did not respond to a request for comment.

The Clinton campaign did not respond to a request for comment on the impact of special interests on the candidate's policies. But, at a forum in New Hampshire last month, Clinton responded to an attack from Sanders about her ties to Wall Street, insisting that contributions and fees she received from speaking at Goldman Sachs events have not influenced her position on regulating the financial sector.

"Anybody who knows me, who thinks they can influence me, name anything they've influenced me on. Just name one thing," Clinton said at the televised forum. "I'm out here every day saying, 'I'm going to shut them down; I'm going after them.'"

The bulk of fossil fuel industry contributions to Priorities USA Action came from two donors. Donald Sussman, the founder and chairman of Paloma Partners, donated $2.5 million. His hedge fund is invested in energy companies like Phillips 66, AGL Resources, and Occidental Petroleum. David Shaw, chief scientist at D.E. Shaw Research, a computational biochemistry research company, gave some $750,000 to the Clinton-affiliated PAC. He also donated $50,000 to Clinton's Ready PAC, formerly known as Ready for Hillary. Shaw served on the President's Council of Advisors on Science and Technology (PCAST) under presidents Bill Clinton and Barack Obama. His investment management firm D.E. Shaw & Co., LP has major holdings in Marathon Petroleum Corporation.

As well as holding substantial assets in oil and gas, Shaw is also an investor in solar energy projects and an ambitious offshore wind project in New England. Greenpeace said that the split in such investments signals a concerning incongruity in interests, especially given Shaw's role in PCAST, which has previously submitted recommendations to the president to continue efforts to decarbonize the economy and invest in renewable energy.

"David Shaw has hundreds of millions of dollars riding on the continued growth of the fossil fuel industry," said Coleman. "That would seem to be at odds with the rapid de-carbonization of the economy necessary to address the threat of climate change, in spite of any opportunistic profit that might be made on the growing wind energy industry."

Shaw did not respond to a request for comment.

Early last year, Greenpeace USA asked both Republican and Democratic 2016 presidential candidates to reject contributions from the fossil fuel sector. Sanders was the only candidate to agree to the pledge. Clinton did not support it, but said she was committed to seeing the reversal of Citizens United, the 2010 Supreme Court ruling that allowed unlimited spending by corporations in candidate campaigns.

Clinton spokesman Josh Schwerin did not respond directly to a request for comment on fossil fuel contributions to her campaign. He said the former Secretary of State has "fought against fossil fuel interests for decades."

"On this campaign, she has laid out tangible, ambitious goals to make America the clean energy superpower of the 21st century," he wrote in an email. "The more than 850,000 people who have contributed to her campaign know exactly where she stands on these issues."

At the democratic debate in Flint, Michigan on Sunday, CNN moderator Anderson Cooper asked Sanders if he believed Clinton was "in the pocket of the fossil fuel industry." The question stemmed from a statement made by the senator the previous week. He said, "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 denied the suggestion that he thought Clinton was in the pocket of the oil and gas industry, but condemned her acceptance of donations from special interests in general.

"Instead of standing up to that finance system, Secretary Clinton has a super PAC, which is raising ... a lot of money from Wall Street and from the fossil fuel industry," Sanders replied.

Greenpeace's Coleman emphasized that there is not a clear distinction between Wall Street and fossil fuel interest contributions, as shown in the data on donations to Priorities USA Action.

"Our research has shown that often, Wall Street and fossil fuel interests are one and the same," Coleman said. "Some of Hillary Clinton's biggest Super PAC donors are Wall Street investors who have invested hundreds of millions of dollars in fossil fuel extraction."

He added, "While there may not be much difference between what Wall Street investors want from a Clinton presidency and what major fossil fuel interests might want, it is safe to say that fossil fuel industry interests rise to the top of the list of Clinton donors."

At Sunday's Democratic debate, Cooper asked Clinton about her appearance at a fundraiser in January co-hosted by Michael C. Forman, the founder and CEO of Franklin Square Capital Partners, a $17 billion investment firm with significant stakes in several Pennsylvania fracking companies.

Clinton responded that she did not "believe that there is any reason to be concerned" about attending the fundraiser and instead emphasized her ambitious climate change plan, which includes a plan to generate 33 percent of the nation's energy from renewable sources by 2027, and to add half a billion solar panels by the end of her first term as president. Clinton estimates her renewable energy policies will cost around $60 billion over the next decade, which will be paid for in part by ending tax breaks for oil and gas companies.

Clinton says she supports — and will expand upon — Obama's regulation of power plant and vehicle emissions. But she recently backtracked on a commitment to impose on her first day in office a moratorium on coal, oil, and natural gas drilling on public lands. She has pledged to support protecting the Arctic and is "very skeptical" about the possibility of oil and gas drilling off the South Carolina coast. Under her leadership, the US Department of State lobbied against fracking bans in Eastern Europe and was widely criticized by the environmental movement and climate change activists for her delay in opposing the Keystone XL pipeline.

During Sunday's debate in Flint, a University of Michigan student asked Clinton directly, "Do you support fracking?"

"I don't support it when any locality or any state is against it, number one," she said. "I don't support it when the release of methane or contamination of water is present. I don't support it — number three — unless we can require that anybody who fracks has to tell us exactly what chemicals they are using."

She added, "By the time we get through all of my conditions, I do not think there will be many places in America where fracking will continue to take place."

For his part, Sanders responded by saying, "No, I do not support fracking."

Throughout her campaign, Clinton has emphasized boosting renewable energy production, rather than waging a full-frontal assault on fossil fuels.

Clinton's renewable energy plans hue closely to those of Tom Steyer, the billionaire hedge fund manager and environmentalist who spent nearly $74 million in the 2014 midterm elections. Steyer is a longtime Clinton supporter who held a $2,700-a-head fundraiser for her at his San Francisco home last year. He has pledged to only support candidates whose environmental policies ensure 50 percent of the nation's electricity is produced by renewables in 2030 and 100 percent by mid-century — a benchmark that Clinton's clean energy plan exceeds.

Steyer did not respond to a request for comment on fossil fuel contributions to Clinton-linked super PACs and to Clinton directly.

Greenpeace insists that Clinton needs to "take a firm stance and be a leader on the issue of money in politics."

"Because of nature of super PACs, we don't know what these donors are being promised or what these donors want for their cash," said Coleman. "If she's going to be our candidate and really believes these ideals, she really needs to take leadership position on this, or it's not going to happen."