BOSTON (AP) — Democratic Senate hopeful Edward Markey is continuing to press Republican rival Gabriel Gomez to sign a pledge intended to discourage outside groups from pouring money into campaign ads in Massachusetts’ special U.S. Senate election.

Markey sent a letter to Gomez on Friday asking that he join him at the Omni Parker House in Boston on Monday to sign the so-called “People’s Pledge.”

READ MORE: Amazon Driver Charged With Child Porn After Allegedly Wearing Wig, Taking Pictures In Wrentham Outlets Bathroom

Gomez, who has refused to sign the pledge, responded by demanding Markey return the millions he’s received from political action committees during his three decades in Congress.

RELATED: Poll Shows Close Race

READ MORE: 5-Month Old 'Fenway Baby' Becomes Crowd Favorite At Red Sox Game

Gomez also said Markey should donate to charity the more than $1 million spent by outside groups on his behalf during the just-completed Democratic primary, which Markey won.

Republican Scott Brown and Democrat Elizabeth Warren signed a similar pledge last year.

The special election in June 25.

MORE NEWS: Suspect Arrested After 7-Hour Standoff In Boston

Copyright 2013 The Associated Press.