Over the past three weeks, the American public has had the chance to hear almost every Democratic candidate for president talk extensively about climate change and how each would deal with the growing crisis. While the candidates' policy ideas differ, they've shown a Democratic Party which, across the political spectrum, is remarkably aligned with the science on the mid-century goal and committed to transforming the economy with more urgency than the Obama administration ever publicly considered.

The Democrats have united around the goal of driving carbon emissions from fossil energy to net zero by mid-century. That includes the moderates and low-polling candidates who had their first chance to detail their ideas on MSNBC's climate forum Thursday and Friday.

It was a pronounced refrain on a week when millions of young people around the world were making their voices heard in a global climate strike, and when teenage climate activist Greta Thunberg appeared before a U.S. congressional committee with a simple message: "Don't listen to me. Listen to the scientists," she said, submitting a copy of the United Nations' Intergovernmental Panel on Climate Change report on what it would take to keep global temperatures from rising more than 1.5 degrees Celsius.

That target has been taken as a given among those seeking the Democratic nomination—a far cry from Obama's carefully hedged goal of lowering carbon emissions 80 percent below 2005 levels by mid-century.

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The challenge that all the Democratic candidates seemed to be grappling with was how they would rally the needed public support behind such a profound energy and economic transformation—especially among the ranks of the working class who have been part of President Donald Trump's base.

Trump's re-election campaign, while not acknowledging that the MSNBC climate forum (to which he was invited) was taking place, sent out press releases seeking to put its spin on the event: "Democrats Aim to Destroy America's Economy," said the headline. It's a message that whomever emerges from the crowded Democratic field will have to be able to counter in 2020.

Montana Gov. Steve Bullock, one of the few Democrats in the field with a record of winning over "red state" voters, talked on Friday about achieving bipartisan buy-in on climate and environment issues in his own state, where he said ranchers might not fully believe humans are causing climate change but they see the changes taking place around them.

"Once you get outside the politics, the values people share are the same," Bullock said. "We should be talking about those values in how we can make this transition." That means talking about clean air and water, how the next generation can do better than the last, and, importantly, about opportunities, Bullock said. Because "it's hard to care about the end of the world when you can't make it to the end of the month."

He sees the jobs and business opportunities that are inherent in the clean energy transition as the key to winning over supporters. A federal climate infrastructure bill would create those opportunities and help unleash American ingenuity, he said.

Sen. Bernie Sanders of Vermont repeated a line he often uses on climate—fossil fuel workers are "not my enemy," he said, clearly an appeal that is meant to underscore the contrast with his 2016 rival, Hillary Clinton, who stumbled badly with her vow to put the coal industry out of business. Although Sanders feels the same about coal, he is trying to enlist Trump's base voters in his drive. Displaced coal and oil industry workers would receive five years of income replacement and health care under his plan, in addition to training to nab some of the 20 million jobs he said his plan would create.

Sen. Cory Booker of New Jersey, with a less combative manner than Sanders, sought to convey that he has similar high ambition. "We need leaders that are unafraid to tell the truth and take the consequences, and this is really one of those moments," Booker said.

Joseph Biden, Elizabeth Warren, Kamala Harris and Beto O'Rourke all declined to participate in the MSNBC forum due to scheduling conflicts. They and several other candidates in the forum, including Andrew Yang, Pete Buttigieg and Julián Castro, had appeared on CNN's climate crisis town hall on Sept. 4.

[Read coverage of the Sept. 4 climate crisis town hall here]

Differences on One Fossil Fuel: Natural Gas

Rep. Tim Ryan of Ohio and Sen. Michael Bennet of Colorado, both more moderate than Sanders or Booker, aimed their appeals at the heartland, but in a transition that includes the fossil fuel industry—at least, natural gas.

Ryan, whose congressional district is on the edge of the largest gas fracking region in the nation, the Marcellus Shale, spoke of natural gas a "bridge" fuel, while stressing the work that needed to be done "on the other side of that bridge." Bennet said "natural gas has a role to play."

Methane leaking from the nation's oil and gas fields has raised questions about whether natural gas can be an effective bridge to lower emissions, and the issue has become a fault line dividing the Democrats.

Ryan returned again and again to agriculture throughout his hour-long interview, and the need to teach new techniques to farmers to curb the heavy use of pesticides and fertilizer. "Regenerative farming is one of my favorite things to talk about," he said.

Bennet said the biggest distinction of his plan is conservation and a focus on incentivizing private landowners to help address climate. "Let's build a coalition of conservation folks, ranchers and other people concerned about our water and our land," he said. "And frankly get to a point where we're actually paying people to sequester carbon in their soil."

Marianne Williamson spent much of her time on the stage talking about the need for a "revolution in consciousness" and exerting "moral influence" to change the "massive state of denial" on the political right.

In marked contrast with Sanders, she cautioned against strong, mandatory policies and "imposing a plan on people" that could anger Republicans and the fossil fuel industry. "People constrict when they feel attacked," she said. "We have to make a moral argument that is so great and talks to all sectors of society that we have buy-in."

Entrepreneur's View on Spurring Innovation

Entrepreneur and former Maryland Congressman John Delaney focused heavily on the innovation he said would be spurred by his carbon fee-and-dividend program, an escalating charge per ton of carbon emitted, which would be returned to households via an annual dividend.

A reflection of how deeply the mid-century zero-emissions goal has embedded in the Democratic agenda is that even Delaney, with his market-based approach to the climate problem, backs it. He talks about the need to focus research on carbon sequestration—including direct air capture and agriculture.

"We can't be sitting around in 2045 and saying, 'oh my gosh, we're not going to get there, what are we going to do?'" he said.

The MSNBC forum, hosted by Chris Hayes and Ali Velshi, was the first time that hedge fund entrepreneur Tom Steyer has had an opportunity to make his climate-focused presidential pitch before a national audience.

"It's been like peeling an onion to understand how to solve this crisis," Steyer said. He said he once thought that investing in research and development was the key to climate action. But he came to realize that political action would be essential.

"In 100 years, people are going to look back and say, 'How could those people have been so brain-dead that they didn't deal with this issue when it was right in front of them?" Steyer said.

A Republican Voice on Climate Action

One indicator of how thoroughly the science is informing the presidential race is that the sole Republican to appear at the MSNBC forum, former Massachusetts Gov. William Weld, talked about the inadequacy of the only climate action bill that has been passed by a chamber of Congress—the 2009 Waxman-Markey bill, which was passed by the House but never take up by the Senate.

"We're going to have to do a lot more than that," Weld said. He said that alongside the federal deficit, climate change is his top issue, and he favors a carbon tax for spurring the change to clean energy. Weld acknowledged that his views have put him out of step with much of his party.

"The Republicans are dug in, saying this is about how we can't have steaks, we can't have airplanes," Weld said. "That's baloney. This is not a sacrifice. This is something we have to do. This is an investment in sustaining the future of the human race on this planet."