“This is about consequences,” Steyer told POLITICO during an interview at Boston’s Cathedral Church of St. Paul, after days of meeting with college students, faith leaders, environmentalists and clean energy executives to map out his role in the Senate campaign. “If you have a pattern of voting for subsidies for oil and gas and voting against renewables and all this other stuff … there have to be consequences. That’s the whole point of this exercise.”…

Some Democratic operatives fret that Steyer and other mega-donors could set up their party for a repeat of what happened to the GOP in 2010 and 2012, when ideologically fueled Senate primary challengers backed by super PACs and other outside groups defeated more mainstream Republicans before losing in general elections.

Noting that several Democratic senators considered vulnerable in 2014 are pro-Keystone — including Max Baucus of Montana, Mark Begich of Alaska, Kay Hagan of North Carolina and Mary Landrieu of Louisiana — one Democratic Senate campaign strategist painted a bleak worst-case scenario.

“Let’s say Steyer causes primary waves and weakens these candidates, and we lose the Senate. Then the environment is even more imperiled than it is right now,” said the strategist, who did not want to be identified questioning the judgment of a mega-donor. If spending by the Steyer-funded NextGen Committee super PAC appeared to set the stage for such a scenario, the strategist predicted, party elders would step in to try to reason with him.