If a Democrat wins the Massachusetts governorship in 2018, natural gas companies should not expect an ally in office.

All three Democrats challenging Republican Gov. Charlie Baker said they generally oppose the construction of new natural gas pipelines.

"We don't need to lock in fading fossil fuel technology for 40 years when everyone else trying to get rid of it," said environmentalist and entrepreneur Bob Massie.

The Democrats are Massie, Newton Mayor Setti Warren and former health insurance executive and state budget chief Jay Gonzalez. The Republican/MassLive.com is running a series of stories looking at the similarities and differences between the Democratic candidates on a variety of issues facing the state, based on interviews with each candidate.

Baker last year signed a law letting the state solicit long-term contracts for wind and hydropower, positioning the state to get more of its energy from renewable sources.

On energy, the Democrats say Massachusetts is poised to become a leader in renewable energy technology.

Massie's expertise is in energy and the environment, having led two companies focused on sustainability and the environment. He believes the state should go further in developing renewable energy sources like offshore wind.

"A country like Denmark just about hit 50 percent (of its power) from wind, we barely are just starting the leasing process for offshore wind," Massie said. "We need to catch up with the rest of the world on technology."

For example, Massie said there is more Massachusetts can do with fuel cell storage, ground source heat pumps, solar energy or systems that draw energy from water or sewer flow.

Massachusetts last year passed legislation to encourage long-term contracts for hydropower and wind. Massie said, "We should be increasing the speed with which we issue grants and permit those different fields."

Massie opposes building new natural gas lines, including the one now under construction that passes through Otis State Forest in the Berkshires. He argued that natural gas endangers the areas a pipeline passes through, but the gas is often sent overseas. He argued that there is no need for more natural gas if the state moves quickly on renewables, which are seeing their prices drop quickly.

Gonzalez listed climate change as "probably the biggest threat to our planet." He argued that Massachusetts should be moving more aggressively toward renewable energy than Baker is doing.

Gonzalez supports an effort to impose carbon pricing in a way that is revenue-neutral and does not impose additional taxes on low- and middle-income individuals.

"While not being a silver bullet, it would be a really important lever toward getting us to a truly clean energy economy," Gonzalez said.

Gonzalez said Massachusetts could be a leader in developing clean energy jobs.

Gonzalez also opposes any expansion of natural gas pipelines. "We don't need additional capacity, and it's just going to further our dependence on fossil fuels," he said.

Warren also said Massachusetts should become a national leader in renewable energy, including wind and solar. He praised the state's net metering credit program, which reimburses homeowners for solar energy they generate but do not use.

He stressed the work he did installing community solar projects as mayor of Newton, and said those types of municipal projects should be done statewide as a way to help low-income people get the financial benefits of solar energy.

Warren said he opposed efforts by the Baker administration to reduce the value of solar credits. Warren also supports lifting the net metering cap.

Warren said Massachusetts can also be a leader in promoting technology to adapt to and prepare for climate change. "I think we have an opportunity to really ramp up Massachusetts as a place to promote climate resilience technology and infrastructure," Warren said.

He too generally opposes the expansion of gas pipelines.