Rep. Donna Edwards (D-Md.) faces an uphill financial battle in her Senate primary. (Aaron C. Davis/THE WASHINGTON POST)

Rep. Donna Edwards has decried the rise of fundraising by outside groups on behalf of candidates from both sides of the aisle, saying that the ability to raise unlimited amounts is “equal-oportunity corrosion.”

But the Maryland Democrat has rejected a pledge to keep such campaign cash out of her primary race for a U.S. Senate seat.

For Edwards, what might seem a tough decision ideologically is also a necessity for political survival. Vastly outraised by Democratic primary opponent Rep. Chris Van Hollen, the four-term member of Congress says she will need spending from outside groups to stay competitive in her bid to succeed retiring Sen. Barbara A. Mikulski (D).

Van Hollen, a prolific fundraiser and former chairman of the Democratic Congressional Campaign Committee, has $3.75 million ready for the race, according to the most recent campaign filings. Edwards has $419,000.

Van Hollen, of Montgomery County, raised $1.5 million in the second quarter of 2015; Edwards, of neighboring Prince George’s County, raised $590,000 and spent $492,211 — mostly on building the kind of fundraising apparatus that Van Hollen has spent years developing.

Donna Edwards and Chris Van Hollen

So when Van Hollen proposed a joint agreement to keep super PACs and other independent fundraising groups from spending money in the race, Edwards quickly rebuffed him.

She said she needs the ads and field organizing that groups like Emily’s List are expected to generate on her behalf, even though it is exactly that type of spending that she, Van Hollen and others have strongly criticized since the U.S. Supreme Court declared it legal in 2010.

Edwards pointed out that the pledge would not forbid donations from traditional political action committees, which in past campaigns have given generously to Van Hollen. Nor would it limit individual donations — Van Hollen had 1,241 itemized donations in the second quarter, compared with 401 for Edwards, and many of his donors gave the maximum allowed by law.

“Van Hollen is raising so much money from so many big interests,” Edwards said. “The idea that I could be foreclosed from receiving the support of pro-choice women, the support of workers is just laughable.”

The first candidates to forgo outside money in a nationally prominent political race were Elizabeth Warren (D) and Scott Brown (R), competing in Massachusetts for a Senate seat in 2012. Because outside groups are required by law not to coordinate with the candidates they hope to benefit, the candidates could not forbid the groups to raise money. Instead, each candidate agreed to donate to a charity of the opponent’s choice an amount equal to any outside funds spent on his or her behalf.

Democratic candidates in Rhode Island’s gubernatorial race agreed to a similar pact last year. In other campaigns, however, including last year’s Democratic gubernatorial primary in Maryland, candidates ignored or turn down such requests by their opponents.

Earlier this month, Van Hollen asked Edwards to agree to a pledge. It took her just a few hours to decline.

She and the groups working on her behalf argued that the pledge would unfairly benefit Van Hollen because he has more direct funding, while Edwards has more support from independent progressive groups.

Emily’s List, which endorsed Edwards in March, is affiliated with both a super PAC, which can take unlimited funds from disclosed donors (including corporations and unions), and an advocacy nonprofit group, which does not have to reveal its donors but is meant to focus on issues rather than politics. Both groups would be barred from helping Edwards under the pledge.

“We’ve always known that we were going to be outraised on this one,” said Jess McIntosh, a spokeswoman for Emily’s List. The pledge, she said, would be cutting off “one of the best ways that she’s going to be able to compete with the corporate lobbyists. . . . I think people see through that.”

The pledge would not prohibit direct donations to Edwards’s campaign by supporters of Emily’s List. Nor would it prevent donations from labor-affiliated PACs or money and time donated by union members.

But Van Hollen is raising several times what Edwards is taking in from PACs and individual donations.

“It’s easy for him to say, ‘Let’s keep out outside groups,’ ” said Donald F. Norris, director of the School of Public Policy at the University of Maryland Baltimore County. “If he continues to have that kind of financial advantage, it will be very difficult for her to maintain a campaign at all.”

Van Hollen said the pledge had nothing to do with political advantage.

“I think that candidates should lead by the power of example here, and this is an opportunity to do it,” he said. “There are lots of people on the outside who would want to help the Van Hollen campaign, but my view is that it’s been bad for our democracy.”

Edwards said her donors tend to be less wealthy than Van Hollen’s, which is why she might need outside help. “I was not the chair of the DCCC. . . . I can’t pull down as many $2,700 donors as Van Hollen,” said Edwards, referring to the maximum donation an individual may give in either the primary or the general election.

She and Van Hollen have both been fighting “dark money” for years.

Edwards was the first member of Congress to propose a constitutional amendment overturning Citizens United v. Federal Election Commission , the court decision that dramatically expanded the ability of corporations and unions to spend on elections. She leads a House Democratic task force on election reform. Van Hollen is behind legislation and a court case aimed at disclosing the groups and donors behind outside advertising.

Both candidates support fellow Maryland Democratic Rep. John Sarbanes’s push for public campaign financing, which would give public funds to candidates who agree to cap private donations.

But Edwards says those efforts are distinct from her current campaign, in which the dollars spent on her behalf could help offset Van Hollen’s overflowing coffers.

Common Cause, a liberal group on whose board Edwards once served, came out with a statement urging Edwards to sign the pledge. Still, president Miles Rapoport acknowledged that such demands put candidates — especially underdogs — in a difficult position.

“Requiring candidates on their own, in the middle of a heated race, to deal with this flood of outside money is unfair,” he said. “All of that should be illegal by law.”

Edwards has continued to court a national base, aware that she has a far smaller network within Maryland than does Van Hollen, who was a state lawmaker for 12 years and has been in Congress since 2003. Early this year, Edwards received a donation from Barbra Streisand; in the second quarter, she took in $2,700 from director and producer J.J. Abrams.

This month, Edwards attended Netroots Nation, an annual gathering of progressive activists. She spoke on the first night of the event and then joined the crowd for karaoke, where she sang “I’m Every Woman.”

Edwards is also spending heavily to expand her donor base. Last quarter, her campaign paid $71,553 to Rising Tide Interactive, a company that helps candidates build their fundraising lists. Her campaign says she’s pushed her donor list from 10,000 to 90,000.

Van Hollen spent $400,000 last quarter, including $9,000 to the Maryland Democratic Party for list acquisition.