(CNN) – Two Democratic congressmen vying for U.S. Senate in Massachusetts have signed on to a pledge calling for super PACs and other third-party groups to stay out of their contest, a pledge also signed by the candidates in last year's Bay State Senate fight.
Reps. Stephen Lynch and Ed Markey endorsed the agreement and encouraged any potential Republican candidates to also sign on.
While the agreement doesn't have any legal ramifications, it stipulates that if outside groups air ads on behalf of a candidate, that candidate will pay half the advertising cost to a charity of his opponent's choosing. Television, radio and direct mail advertising are included in the pledge.
A similar agreement was signed last year by Sen. Elizabeth Warren and former Sen. Scott Brown. While their race was devoid of third-party advertising, it wasn't lacking in nastiness from the candidates themselves, who sniped over Brown's connections to Wall Street and Warren's Native American heritage.
"Outside money has no place in the Massachusetts Senate race," Markey said in a statement. "This election should be focused on issues, not outside-group attack ads."
Lynch added that the race "should be decided in debates and on the stump, not by third-party advertisements or special interest mailers."
Lynch and Markey are competing to take the seat vacated by Secretary of State John Kerry, who officially took over as the country's top diplomat at the beginning of February.
Two Republican candidates have emerged: businessman Gabriel Gomez and State Rep. Dan Winslow. To appear on the ballot, the candidates must collect 10,000 signatures by February 27. A primary will be held April 30 to select a party candidate for the June special election to follow Kerry.