BOSTON (AP) - U.S. Rep. Edward Markey is asking a California billionaire to stay out of the U.S. Senate race in Massachusetts.

Markey said in a statement Friday that Thomas Steyer should immediately withdraw his threat to unleash negative attacks against Markey's Democratic rival, U.S. Rep. Stephen Lynch, unless Lynch denounces the Keystone XL Pipeline by "high noon."

Steyer and other critics of the proposed pipeline from western Canada to Texas say it will carry "dirty oil" that contributes to global warning. Lynch supports the project, while Markey opposes it.

But Markey says Steyer's tactics have no place in the campaign and should be "repudiated."

Lynch has said he will not be intimidated by the demands.

Steyer's supporters in Boston plan to launch what they call an "aggressive" campaign against Lynch on Friday.