Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE declared Tuesday that climate change is the largest national security threat facing the United States.

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“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 that we’re going to be leaving our kids and our grandchildren may well not be inhabitable,” the Vermont Independent said during the first Democratic presidential debate in Las Vegas.

“That is a major crisis,” he said to strong applause and cheering from the crowd in attendance.

Former Maryland Gov. Martin O’Malley also mentioned climate change as one of the top national security threats, saying it “makes cascading threats even worse.”

Sanders and O’Malley have been the most vocal advocates on the 2016 campaign trail of strong action to fight climate change and cut back dramatically on fossil fuels.

Both have set goals of getting half of the country’s electricity from renewable sources by 2030 while cutting back on offshore oil and natural gas drilling, among other pledges.

O’Malley has repeatedly highlighted global warming as a top security threat, inviting mockery from Republicans and others. But he has doubled down on his statements, even saying that climate change is partly to blame for the rise of the Islamic State in Iraq and Syria.

President Obama has made similar assessments. He controversially declared earlier this year that climate change is the worst threat to future generations.

At the Democratic debate, party front-runner and former secretary of State Hillary Clinton said that the spread of nuclear weapons and nuclear material is the biggest security threat for the country.

Former Rhode Island Gov. Lincoln Chafee said Middle East chaos is the largest threat, and former Sen. Jim Webb (Va.) listed China, cyber warfare and Middle East strife as the worst threats.

All candidates except Webb mentioned the need to fight climate change in their opening statements.