Seattle, Washington (CNN) - With Sen. Patty Murray looking vulnerable for the first time in her 18-year career, outside groups have pumped millions of dollars into the state of Washington to influence a marquee race that might ultimately decide which party controls the Senate next year.
Among the big-spenders here: American Action Network, a Republican-friendly group that's spent roughly $1.3 million attacking Murray with TV ads criticizing her homespun "mom in tennis shoes" image, and Crossroads GPS, the free-spending arm of American Crossroads, an independent group with ties to the U.S. Chamber of Commerce and being advised by GOP heavyweights Karl Rove and Ed Gillespie.
Crossroads GPS has spent $1.3 million on television ads in Washington, including one spot on the air accusing Murray of increasing the national debt by $14 trillion with her votes since joining the Senate in 1993.
Both American Action Network and Crossroads GPS groups are Washington, D.C.-based 501(c)4 non-profit organizations that can collect unlimited sums and aren't required to disclose their donors - a circumstance that's drawn scorn from Democrats, including President Obama, who claim the non-profits are "shadowy" front groups for entrenched business interests.
Independent Democratic outfits - including labor-backed groups and an organization called Common Sense Ten –have weighed in on the race to attack Murray's Republican opponent, Dino Rossi, but most of them are required to report the sources of their funding.
One advocacy group backing Murray, however, is just as secretive as the ones President Obama has taken to blasting on the campaign trail.
An organization called The Citizens and Strength and Security Action Fund has spent north of $640,000 on Murray' behalf but, as a 501(c)4 organization like Crossroads GPS and American Action Network, is not required to disclose its funding sources.
Asked about the group's support, Murray spokeswoman Julie Edwards emphasized that the Senator believes all independent political organizations should disclose their donors.
The conservative groups, meanwhile, have far outspent their left-leaning counterparts on the Washington airwaves.
Rossi, who trails Murray by eight points in a new CNN/TIME/Opinion Research Corporation poll of likely voters released this week, tiptoed around the issue of outside spending in an interview Wednesday.
He blamed the rising tide of independent money on legislators who supported the 2002 McCain-Feingold legislation that put strict limits on campaign spending and subsequently drove political money into 527 organizations not regulated by the Federal Election Commission.
"Sen. Murray voted for this, and allowed this to happen in the end, campaign reform," Rossi told CNN. "It ends up pushing money out so it isn't transparent into third party groups. I'd rather see more of it come to the candidates so they can control their own message but they also have to be accountable for their own message and where their money comes from."
The independent groups spending heavily on Rossi's behalf, though, are not 527s.
Two recent Supreme Court rulings, the 2007 Federal Election Commission v. Wisconsin Right to Life, Inc. decision and this year's Citizens United vs. Federal Election Commission decision, struck down key provisions in McCain-Feingold and expanded opportunities for groups like American Action Network and Crossroads GPS to raise money and spend freely so long as they don't coordinate with any candidate.
Pressed on whether he welcomes the support from the outside groups who don't reveal their donors, Rossi answered: "It's their First Amendment right and I have no control over it. Clearly it's coming from both sides."
Edwards, the Murray spokeswoman, scoffed at Rossi's comments.
"The only thing transparent about Mr. Rossi's position is his consistent support for secret donations from Wall Street hedge fund managers who fund his campaign and the baseless attacks against Patty Murray," Edwards told CNN. "He is clearly grasping for any excuse to keep the spigot of secret donations open."
Murray co-sponsored and voted twice for the DISCLOSE Act, a bill that would require corporate and union-backed groups to disclose their donors, but the legislation fell short of the 60 votes needed to overcome a Republican filibuster last summer.
Rossi spokeswoman Jennifer Morris said in an email that he would not have supported DISCLOSE if he were in the Senate, citing "special interest loopholes" and "incumbency protection" built into the legislation.
Money aside, Murray is clearly facing the toughest battle of her long Senate career, dragged down by the slouching economy and a ruthless political environment for incumbents, even in a state that Barack Obama carried by 18 points in the 2008 presidential election.
Rossi has the added benefit of being the most famous Republican in the state, having twice run unsuccessfully for governor. In 2004, he famously lost to Democrat Christine Gregoire in a dramatic recount by just 133 votes.
Murray is pushing back against Rossi's charges that she's lost touch with her state since her first Senate win in 1992, calling herself "the tough mom who comes home every weekend."
"I am out in the communities talking to people, and I hear their stories and I take them back with me to make sure the policies of Washington, D.C. reflect the values of the people of Washington state and work for them," she told CNN.
Rossi says Murray backs higher taxes that will snuff out small business growth and has chided her unabashed support for federal earmarks (Murray likes to call them "targeted investments").
"We are finding folks who are just willing to say, you know, we've got to go in a different direction," Rossi said. "If you like her bailouts and her government-controlled health care and her stimulus and her earmarks, vote for Sen. Murray, she's going to give you six more years of the same."