Two Bills. Two days. Two different views on Colorado’s most controversial ballot measure.

Soon after Colorado starting mailing out election ballots, the debate over perhaps the most contentious measure state voters must decide — Proposition 112 — was taken up in two different communities by two prominent men, both named Bill.

First was Bill McKibben, the nationally known writer and climate change crusader, who was in Englewood for a fundraiser sponsored by his organization, 350.org. The next morning Bill Ritter, a former governor of Colorado and founder of the Center for the New Energy Economy, held forth at the Fort Collins Book Festival.

Both Bills agree climate change is real and urgent. Both argue for science-based public policy. But they disagree on the response, and, specifically whether Proposition 112’s proposed 2,500-foot setback from homes, schools and other “vulnerable” locations is the right response to global warming.

“The fossil fuel industry … wants to go as slowly as possible so they can make the last 10 cents out of their business model, even if they have to break the planet to do that,” McKibben said.

For the foreseeable future, Ritter said, “We actually need natural gas to be part of our energy fuel mix.”

McKibben, 57, has been a nagging, influential voice about climate change since the 1980s. He’d arisen at 3 a.m. the morning before at his home in Vermont, flown to California to support a ban on drilling in San Luis Obispo County, then arrived in Colorado to help work the telephone banks and to induce financial pledges. He succeeded. There were whoops and hugs in the evening’s come-to-Jesus session as donors pledged $233,000 for advertising — chump change, attendees were told repeatedly, compared to the oil and gas industry’s reported $35-million war chest.

The happy huzzahs of 350.org’s fundraiser the previous evening were entirely absent when Ritter stepped onto the podium in a forum overlooking the Poudre River. Admission was free, but most seats sat empty.

Ritter, 62, had declared his opposition to Proposition 112 only a few days before in an op/ed in The Denver Post.He reminded readers that upon becoming governor in 2007 his first major step was to more strongly regulate oil and gas drilling. “It took the better part of two years, and some difficult battles with the industry, to complete that overhaul,” he wrote. “When that process was complete Colorado had the toughest set of regulations for oil and gas development in the country.”

In Fort Collins, Ritter said no scientific studies support a 2,500-foot setback as opposed to, for example, a 1,000-foot setback. State law currently requires 500 feet from homes and 1,000 feet from places of congregation, including schools and hospitals. Rule-making should, he said, be based on science.

Ritter also said that the proposed setbacks would put 80 to 85 percent of Colorado off-limits to drilling and likely put thousands of people out of work.

McKibben, in his 20-minute talk, made it clear that he believes climate change is the giant, hulking issue. The son of a newspaperman in Boston, he went to Harvard and soon after began writing for the New Yorker. He frequently has articles about climate change politics in Rolling Stone, The Nation, and other left-tilting magazines.

Last Sunday, an op-ed in the New York Times titled “Let’s Agree Not to Kill One Another,”he wrote about the breakdown in civility, the death threats he sometimes gets, about the need to be able to sit down and talk over issues. In that, his language resembles that of Ritter.

McKibben says he’s driven by the melting ice in Greenland, the rising seawater in the Marshall Islands, and more. “This is no longer some abstract theoretical trouble that we’re facing. It is a trouble that is hitting some place around the world every single day,” he said.

He sees no time to wait. “The arc of the physical world is short,” he said in a twist on the famous Martin Luther King quote, “and it seems to bend toward heat.”

Hydrocarbons, said McKibben, “need to be kept in the ground.”

Where McKibben bites with pessimism fringed by wry humor, Ritter holds forth straight-forward optimism. In Fort Collins, he pointed to the pivot of Atlanta-based Southern Company, one of the nation’s largest utilities. As recently as 2007 it generated 71 percent of its electricity from coal, but now aspires to be emissions neutral by 2050.

Ritter grew upon a wheat farm east of Denver, the son of a construction heavy-equipment operator. He went to Colorado State University. In the late 1980s, as McKibben was writing his first book about global warming, “The End of Nature,” Ritter and his wife, Jeannie, were in Africa, as Catholic missionaries in Zambia, where they opened a food distribution and education center. Leaving the governor’s mansion in 2011, Ritter founded the Center for the New Energy Economy.

Some distrust Ritter’s close ties — he says they’re non-financial — to the natural gas industry. McKibben gets accused of hypocrisy because he flies frequently across the county in gas-guzzling jets to talk about keeping fossil fuels underground.

In Fort Collins, Ritter mentioned that he had contributed to a 2014 National Academies of Science report that recommends doubling down on research and development but also enacting a price on carbon. “We said put a price on pollution,” he explained. He sees policies that unleash market forces being the way to transform energy use. He wants the next governor of Colorado to push for deeper decarbonization of transportation, particularly along Interstate 25 in northern Colorado, where 80 percent of Coloradans live. He calls for a just transition, one that honors  the role of fossil fuel extraction in creating the middle class.

Does the urgency of climate change described by McKibben demand dramatic action — like Proposition 112?  No, answered Ritter. “We agree on the problem, but I think we have different approaches.”

Both Bills also talked about Colorado influencing the national debate, first with the nation’s first voter-mandated renewable portfolio standard, a minimum percentage of renewable energy. Xcel Energy resisted but then blew past the original 10 percent renewable portfolio standard and is now pushing toward 55 percent. McKibben thinks Colorado should set the same example with drilling. Ritter thinks the measure goes too fast, too far.

Two men. Same name. Same belief that carbon must be kept in the ground. Poles apart on how urgently it needs to happen.