A political-action group formed to attack the business record of 1st Congressional District candidate Suzan DelBene was funded entirely by the mother of one of DelBene's opponents, Laura Ruderman, according to Federal Election Commission filings released Sunday.

The PAC, Progress for Washington, plans to press its attack in TV ads against DelBene, thanks to a $115,000 contribution from Ruderman's mother, Margaret Rothschild. Until Sunday, the PAC's donors were not known.

It is the only independent spending thus far in the hotly contested race for the House seat, but it has added a nasty edge as five Democrats vie to advance to November and likely face Republican John Koster.

In a statement, Rothschild, who lives on Vashon Island, said she paid for the ads because she was "very frustrated" that DelBene, a millionaire former Microsoft vice president, could self-fund her campaign while Ruderman could not.

DelBene has donated $1 million to her campaign since March, more than Ruderman and Darcy Burner, another leading Democrat, have raised, combined, from their thousands of donors, according to the FEC filings. DelBene's campaign plans a late-July TV ad blitz, with nearly $730,000 in airtime purchased, the filings show.

The ads Rothschild paid for portray DelBene as profiting from failed startups she led, and tie her to a period in which Microsoft fell far behind Apple in the smartphone market.

Federal election laws limit coordination between campaigns and independent groups that support them to no more than $5,000 for an election cycle.

On Sunday, Ruderman campaign manager Liz Berry said Ruderman learned of her mother's involvement from a reporter, and was "just shocked" that Rothschild has funded the ads.

"She had no idea it was her mother," Berry said. Rothschild is a counselor who "knows how to keep secrets," Berry said.

DelBene said the ads contain "false accusations," and asked Ruderman to denounce them during a joint appearance Friday.

"She had the opportunity, but didn't," said DelBene. "I'm disappointed that she hasn't denounced them."

Ruderman talks about her mother's battle with cancer as part of her stump speech on health-care financing, and Rothschild appears in a video on Ruderman's campaign website. Berry says the mother and daughter talk weekly about Rothschild's cancer treatment.

In her statement, Rothschild said she funded the attack ads "out of love for my daughter," and relied on advice from Jeff Gumbinner, a political consultant who previously worked with Ruderman.

Rothschild's husband, George Heidorn, was a Microsoft senior researcher who frequently donates to Democrats.

The 1st District, which stretches from Redmond to Canada, is an open seat after Jay Inslee quit in March to run for governor. Its boundaries were redrawn last year to make it district with an even split between Democrats and Republicans, and national political handicappers rate is as one of the most-contested congressional races in the country.

According to mandatory campaign filings Sunday, Burner, who has run for Congress twice before, raised $178,119 in the past three months, much of it via the national Act Blue network, which supports liberal candidates.

Ruderman raised $60,000 less but has much more cash on hand — $276,194 — to spend as primary-election ballots begin to be mailed.

Her campaign has spent about $200,000 on TV ads, to begin airing next week.

Koster, a Snohomish County councilman, has remained quiet as the Democrats battle. He has $115,632 on hand as of Sunday, but is likely to be aided by outside conservative groups eager to see a Republican take a seat held by Democrats since 1999.

Another leading Democrat in the race, centrist state Sen. Steve Hobbs, had not filed his campaign reports as of late Sunday but has indicated he raised less than his opponents..

The negative ads by Progress for Washington prompted U.S. Sen. Patty Murray, D-Wash., on Friday, before Rothschild's connection was disclosed, to ask the Democrats to avoid getting to a "messy intraparty fight."

Jonathan Martin: 206-464-2605 or jmartin@seattletimes.com. On Twitter @jmartin206.