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.

Tom Steyer is wading into a special election in Massachusetts. (Jahi Chikwendiu -- The Washington Post)

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.

Markey is telling Steyer to butt out.

"As I stated when I first learned about Tom Steyer's demands on Monday, these kinds of tactics have no place in our political discourse and should be repudiated," Markey said in a statement. "Mr. Steyer should immediately withdraw his threats and ultimatum, and stay out of this Senate race. This campaign should be about the people of this state, and I remain committed to giving Massachusetts voters a debate about the issues they care about most."

Lynch and Markey both signed a pledge to keep outside groups out of the race. Any time an outside group advertises on behalf of one candidate, that candidate must donate half the cost of the ad buy to a charity of his opponent's choice.

Steyer was not moved; his campaign launched Friday afternoon. "The issue in this race is whether Steve Lynch is running to be the Senator from Canada fighting for increasing the wealth of a foreign oil company or a Senator from Massachusetts who will stand for the common good of the Commonwealth," spokesman Chris Lehane said.

Steyer says he will abide by the terms of the pledge, avoiding the television, radio, web or direct mail ads that are barred. His campaign is kicking off with ads on the sides of RVs that will drive around heavily Democratic areas of the state. Steyer is also planning to fund field work, college campus events and phone banking -- none of which is covered by the pledge. (Other outside groups have also exploited these loopholes).

However, according to the Markey campaign, Steyer has broken the pledge by advertising on Facebook.

"We are taking action to notify Steyer's group about the terms of our agreement with the Lynch campaign, and we will make a contribution to the charity of Stephen Lynch's choice as the People's Pledge dictates," Markey campaign manager Sarah Benzing said. "We once again ask Tom Steyer to cease his Facebook advertising campaign, honor our People's Pledge and end his involvement in this Senate race."

The group is advertising on Facebook for its own Facebook page, but not against Lynch.

Lynch has earned the ire of liberals not just for his environmental policies but also for his opposition to President Obama's health-care reform and to abortion rights. Markey has the support of the Democratic establishment, and polling has given him the lead in this race.