Democratic candidate Hillary Clinton recently announced her plan to install half a billion solar panels in the U.S., but the initiative isn’t impressing her primary opponent Sen. Bernie Sanders.

Sanders blasted Hillary for promising people millions of solar panels while not publically stating whether or not she opposes the Keystone XL pipeline — which both Sanders and former Maryland Gov. Martin O’Malley oppose.

“It is hard for me to understand how one can be concerned about climate change but not vigorously oppose the Keystone pipeline,” the Vermont senator said in a statement after Clinton released her green energy plan.

“We have to address the planetary crisis of climate change and there is no question that we must move aggressively toward energy efficiency and the development of sustainable energy sources such as wind, solar, geothermal and biomass,” Sanders said. “So I agree with Secretary Clinton about the need for substantial investment in sustainable energy.

“But that is not enough,” Sanders added. “We must make significant reductions in carbon emissions and break our dependency on fossil fuels. That is why I have helped lead the fight in the Senate against the Keystone pipeline which would transport some of the dirtiest fossil fuel in the world.”

Sanders has opposed Keystone XL for years, earning praise from environmentalists who also oppose the project. Clinton has been criticized by activists for not publically stating her position on the project, instead she’s repeatedly refused to answer questions on the topic.

“This is President Obama’s decision, and I am not going to second-guess him,” Clinton recently told a group of people at a campaign stop. “If it’s undecided when I become president, I will answer your question.”

Luckily for Clinton, she may never have to take up the issue should she be elected president in 2016. The White House plans on vetoing the project during Congress’ August recess, sources told North Dakota Republican Sen. John Hoeven.

Clinton’s refusal to talk about Keystone XL may be angering environmental activists, but her stance has been defended by San Francisco billionaire Tom Steyer. Steyer himself became a “green” icon because of his opposition to Keystone and his spending of millions of dollars backing Democrats and environmental causes.

Steyer has said he won’t demand Clinton take a position on the pipeline. His hesitance to bash Clinton comes after he hosted a fundraiser for her earlier this year in San Francisco — that event was rather ironically protested by environmentalists.

“She isn’t for Keystone,” Steyer told Politico Pro. “She has no position.”

“We haven’t insisted that people make a decision on that because it hasn’t been necessary,” Steyer added, speaking about his campaign group NextGen Climate Action. “Maybe we’ll change our minds. We drew a line in the sand on supporting it, but we haven’t drawn a line in the sand and said that absolutely everybody has to take a position on this.”

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