Hillary Rodham Clinton took more heat Sunday over her repeated refusals to take a stand on the Keystone XL pipeline, this time from Sen. Bernard Sanders of Vermont, her rival for the Democratic presidential nomination.

The former secretary of state declined again last week at a campaign event to say whether she would green-light the enormous pipeline project, saying, “If it’s undecided when I become president, then I will answer your question.”

Asked about her comment, Mr. Sanders said, “Well, listen, needless to say, I have a hard time understanding that response.”

“I have helped lead the effort against the Keystone pipeline,” Mr. Sanders said on ABC’s “This Week.” “And I’m very proud, by the way, that just yesterday we received the endorsement from Friends of the Earth, one of the largest environmental organizations in America. And that is one of the reasons that they gave me their endorsement.”

Mrs. Clinton is losing support among environmental groups over her refusal to comment on the Keystone pipeline, which she has attributed to her work on the issue at the State Department.

Bill McKibben, head of the climate change group 350.org, described her position last week as “bogus.”

“Look, the notion that she can’t talk about it because the State Dept. is still working on it makes no sense,” Mr. McKibben told Mother Jones in an email. “By that test, she shouldn’t be talking about Benghazi or Iran or anything else either. The more she tries to duck the question, the more the whole thing smells.”

Mr. Sanders said his opposition to the pipeline, which would deliver oil from Canada to the U.S. Gulf Coast, is based on his conviction that increased levels of carbon dioxide in the atmosphere are fueling global warming.

“For all intents and purposes, the debate is over, the scientific community is virtually unanimous in telling us climate change is real. It is caused by human activity,” he said.

The senator, a self-described socialist who has surprised politicos with his packed campaign events, drew 100,000 attendees, the largest crowd of the 2016 campaign season, at a national video-simulcast event Wednesday billed as a “house party.”

“We have made phenomenal progress in the last three months,” Mr. Sanders said Sunday. “And you know what, we’re going to continue to make that kind of progress.”

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