A billionaire environmentalist is moving ahead with his campaign against Rep. Stephen Lynch (D) in the special election for Massachusetts's open Senate seat, even as the congressman's primary rival asks him to stay out.

Lynch and Rep. Ed Markey are competing in the Democratic primary for the seat currently held by Sen. Mo Cowan (D). Tom Steyer, a California mogul and philanthropist, has threatened to run an extensive campaign against Lynch unless the congressman drops his support for the Keystone XL pipeline.