PORTLAND, Maine — Half of Maine’s congressional delegation joined Mayor Michael Brennan on Monday at a Monument Square rally urging public support for a new federal plan to counter climate change.

Democratic U.S. Reps. Chellie Pingree and Mike Michaud, Brennan, civic leaders and business people greeted a bus that’s touring 27 states to promote the plan, which President Barack Obama unveiled June 25.

The plan includes a series of executive actions the president is taking to reduce the effects of global warming, including the nation’s first limits on carbon pollution from power plants, and measures to increase energy efficiency and the use of renewable energy sources.

The “I Will Act on Climate” tour, organized by a coalition of health and environmental groups, hit the road July 11 in Nashville, Tenn. The trek ends Aug. 19 in Washington, D.C., after making 49 stops, including in Portland, Bangor and Portsmouth, N.H.

To minimize its own effect on climate change, the bus runs on clean-burning biodiesel fuel, according to Judy Berk of the Natural Resources Council of Maine.

On Monday afternoon, Pingree told a crowd of about 50 people outside the bus that global warming was already affecting Maine’s fishing industry. She mentioned the explosive growth along the coast of green crabs, which has ravaged shellfish populations and is due in part to rising ocean temperatures. The state’s cold waters had previously kept the crabs’ numbers in check.

“This isn’t something we’re talking about theoretically. It’s for real,” Pingree said.

Jim Merryman, a lobsterman for more than 30 years, agreed. “We’re dealing with a fragile environment,” Merryman said, noting that climate change is also a threat to lobster fishing, the “heartbeat of so many coastal communities.”

Merryman is founder of Potts Harbor Lobster in Harpswell. The lobster business and its wharf are the only working waterfront in Maine using solar-generated electricity, according to Merryman. Using solar power, Potts Harbor prevents about 20,000 pounds of carbon dioxide gas from entering the atmosphere each year, he said.

Michaud praised the embrace of clean energy throughout the state, including the commercial development of wind and tidal power sources. “Tackling climate change is an opportunity, not just a challenge,” he said.

Brennan touted the city’s own work to reduce climate change. Over the next few years, the school system will increase the locally produced portion of the food it serves from 30 percent to 50 percent, he said, reducing the need to transport food in carbon-belching trucks. And planned renovations to school buildings will incorporate energy-efficient design.

Brennan said his concern about the earth’s future climate has grown more acute since Aug. 9, when his first grandchild was born. And he compared the future effect of climate change to environmental risks that could have been avoided if appropriate action had been taken in 1970, the year of the first Earth Day.

“If we had taken just one-tenth of the steps we should have, we wouldn’t be in this predicament today,” he said.