Climate change is the greatest threat facing the U.S. today, Sen. Bernard Sanders said Tuesday night at the Democratic party’s first presidential debate.

Each of the five candidates at the Las Vegas forum was asked to rank the No. 1 threat facing the U.S., and while others cited a nuclear Iran, a rising China or the spread of the Islamic State, Mr. Sanders, Vermont independent seeking the White House as a Democrat, said global warming could leave the planet uninhabitable for future generations.

“The scientific community is telling us if we do not address the global crisis of climate change, transform our energy system away from fossil fuels to sustainable energy, the planet we’re going to be leaving our kids and our grandchildren may well not be habitable. That is a major crisis,” he said.

In his own answer, former Maryland Gov. Martin O’Malley also cited global warming, though he ranked it behind other foreign-policy challenges.

“Climate change makes cascading threats even worse,” Mr. O’Malley said.

Later in the debate, former Secretary of State Hillary Clinton said she’s been a leader in the fight against climate change.”I have been at the forefront of dealing with climate change starting in 2009 when President Obama and I crashed a meeting with the Chinese and got them to sign up for the first international agreement to combat climate change that they’d ever joined,” Mrs. Clinton said.

“I’m not taking a backseat to anybody on my values, my principles and the results I get.”

Mrs. Clinton was referencing a meeting at an international climate-change summit in Copenhagen in 2009 during which the U.S. delegation — led by Mr. Obama and then-Secretary Clinton — tracked down Chinese leaders for an impromptu discussion on global-warming policy. She’d previously discussed the incident in her book, “Hard Choices.”

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