Democratic U.S. Senate hopeful Edward Markey is calling on Republican candidates to sign a pledge discouraging outside groups from running political ads in Massachusetts' special Senate election _ but said the ban shouldn't extend to donations from political action committees.

    Markey and fellow Democratic Senate candidate Stephen Lynch have signed the so-called ``people's pledge.''

    Two Republican candidates, Daniel Winslow and Gabriel Gomez, have rejected it.

    Winslow called the pledge ``inauthentic,'' saying Lynch and Markey have filled their campaign accounts with money from outside Massachusetts. He said he'd welcome ``positive'' help from outside groups.

    Gomez said the only pledge he'll take is to defend the U.S. Constitution.

    Markey said PAC donations are different because they are made public.

    Markey and Lynch both collected hundreds of thousands of dollars from PACs during the past two years. (Associated Press)