Rep. Chris Van Hollen, right, wants Rep. Donna Edwards, left, to swear off super PACs. (Left: AP Photo. Right: Washington Post.)

Rep. Chris Van Hollen (D-Md.) wants to keep super PACs out of the Democratic primary for Maryland’s open Senate seat, but his primary rival, Rep. Donna Edwards, does not agree.

In a letter Thursday, Van Hollen asked Edwards to sign a pledge refusing any campaign help from outside groups.

“We can discourage outside groups and deep pocketed individuals from dumping money into Maryland,” Van Hollen wrote. “This should be a race for Marylanders.”

Edwards rejected the pledge, saying in a statement that “until we have real reform, it is wrong to silence ... pro-choice Democratic women, working families and progressive advocates in this campaign.”

Under the pledge, if a third-party group aired any ad or contacts voters in support of one candidate or attacking the other, the candidate who benefits from the spending would donate half its amount to charity. Both would promise “good faith efforts” to keep independent groups out of the race.

Both candidates are deeply involved in efforts to curtail the effects of 2010’s Supreme Court Citizens United ruling allowing greater outside campaign spending. Van Hollen has for years tried to pass legislation expanding disclosure requirements for outside groups involved in campaigns. Edwards supports that effort, though she opposed a 2010 compromise effort designed to exclude the National Rifle Association from new rules. She also chairs a House Democratic task force focused on electoral and campaign finance reform and was the first House member to propose a constitutional amendment overturning Citizens United.

Van Hollen, however, is in a stronger position to reject outside spending. He has raised nearly three times as much money as Edwards thus far. More of his funding comes from within Maryland, while Edwards has leaned on her ties to national progressive groups.

Among those groups is EMILY’s List, a powerful supporter of Democratic female candidates that has the funds to make large expenditures on Edwards’s behalf in the absence of this pledge.

In a statement, the president of the liberal advocacy group Common Cause, Miles Rapoport, said that “all candidates should take the pledge.”

“The pledge enhances the authentic voice of the candidates and significantly decreases the distractions from outside independent advertising whose donors are often secret,” Rapoport said.

At the same time, he called both candidates “champions” on campaign finance reform and did not criticize Edwards for rebuffing the pledge.

No other Democrats have entered the race to replace Sen. Barbara A. Mikulski (D), who is retiring after 30 years in the Senate.

A similar pledge was signed by Sen. Elizabeth Warren (D-Mass.) and her Republican opponent, Scott Brown, in 2012. After losing that race, Brown refused to agree to a new pledge when he ran against New Hampshire Sen. Jeanne Shaheen (D) two years later.

Both Edwards and Van Hollen have sought to claim affinity to Warren, a star among progressives. Edwards has described herself in campaign e-mails as “an Elizabeth Warren ally” and “from the Elizabeth Warren wing of the Democratic Party.” In a statement Thursday, Van Hollen’s campaign manager said that “like Sen. Warren, Chris believes in leading by example.”

Warren did not face a competitive primary in 2012, and the vast majority of outside spending flows in general elections. Massachusetts Democrats attempted unsuccessfully to agree on an anti-super PAC pledge in the 2014 gubernatorial primary.

Rhode Island Democrats did impose such a ban on themselves in their gubernatorial primary that same year. Two Democratic presidential candidates, Sen. Bernie Sanders (I-Vt.) and former Virginia senator Jim Webb, have promised to eschew super PAC help.