The political action arm of, in its first foray into electoral politics, has decided to back Rep. Edward J. Markey (D-Mass.) in his primary race to fill the Senate seat left vacant by John Kerry’s move to head up the State Department.

The group, 350 Action, said that Markey’s opposition to the Keystone XL pipeline was the decisive issue. His primary foe, Rep. Stephen Lynch (D), cast two votes for measures that 350 Action spokesman Daniel Kessler said were designed to force President Obama into supporting the pipeline. Both measures were opposed by the overwhelming majority of House Democrats.

350 Action is the second major environmental group to endorse Markey this week. On Monday, the League of Conservation Voters said it would spend at least $650,000 on a field campaign to rally support for Markey. It cited Markey’s 94 percent record on LCV’s scorecard issues and his role in sponsoring a cap-and-trade bill three years ago.

350 Action and oppose the Keystone XL pipeline because it would carry oil sands from Canada whose energy-intensive extraction emits more greenhouse gases than conventional oil as well as renewable energy sources.

“We’re looking to open up the next front in the Keystone fight,” Kessler said. “As it goes forward, it will be more and more of a congressional fight.”

350 Action is the second major environmental group to endorse Rep. Ed Markey (D-Mass.) this week. (Michael Dwyer/AP)

Kessler said that until now, 350 Action was “very dedicated to building a movement on climate change. Sometimes to do that, you need to be outside politics.” But, he added, “this is such an important issue that we’re willing to engage in electoral politics.”

He said that the group would also attempt to press Sen. Mark Warner (D-Va.) because he recently signed a letter, along with 52 other senators, asking the president to approve the Keystone XL pipeline.

Lynch’s campaign defended the congressman’s record. “Congressman Lynch, like President Obama, thinks we should be looking at all options,” said Conor Yunits, a spokesman for the Lynch campaign, “to protect the environment and to protect our economy and reduce our dependence on foreign energy sources.”

Yunits said that Lynch’s career voting record on LCV’s scorecard was the same as Markey’s.