Scott Brown wasn't even an official candidate when he declined to agree to a "people's pledge" to limit the amount of outside spending in the New Hampshire Senate race.
Six months later, with Brown expected to win the Republican nomination in Tuesday’s primary, more than half of the $19.1 million spent on the New Hampshire race has come from outside groups.
Of the roughly $9.9 million in independent expenditures so far, groups that do not disclose their donors have accounted for more than $5 million - nearly all of it aimed at softening up the Democratic incumbent, Sen. Jeanne Shaheen, for the November general election.
The dark money began to flow in January, a good three months before Brown officially entered the race.
In a series of attacks on Democratic incumbents, Americans for Prosperity, a conservative 501(c)4 and the main political arm of billionaire industrialists David and Charles Koch, took aim at Shaheen's support for the Affordable Care Act.
Since then, AFP has spent more than $2.6 million attacking Shaheen on the airwaves, according to news accounts of the group’s ad buys in New Hampshire television markets.
Another dark money outfit, the conservative 501(c)4 Ending Spending, has spent $1.3 million on anti-Shaheen attacks ads, while Crossroads GPS, founded by Republican political operative Karl Rove, has dropped another $600,000.
The U.S. Chamber of Commerce, which also does not disclose its donors, jumped on Brown's bandwagon last week, launching a $500,000 TV- and web-ad campaign featuring an endorsement from former Republican presidential nominee Mitt Romney.
Brown was the Senator from Massachusetts in 2012, when he proposed a "people's pledge" for his race against Elizabeth Warren. Warren agreed, and outside spending accounted for just over $8 million, or about 9 percent of the total spent.
Brown had far fewer resources when he entered the New Hampshire race - about $153,000, compared to the $3.4 million Shaheen had in the bank - and he called Shaheen's offer to limit outside spending "hypocritical and self-serving."
But, so far anyway, outside groups supporting Democrats have spent just over half - about $2.8 million - of that spent by Republican groups.
Leading the way is NextGen Climate Action, founded by billionaire environmental advocate Tom Steyer, which has spent $1.1 million. The Senate Majority PAC has dropped just over $1 million. Both groups disclose their donors.
One dark money group, the League of Conservation Voters, has pitched in about $376,000 to attack Brown.
Not all of the outside spending has been focused on Brown or Shaheen. The MayDay PAC, an anti-corruption group founded by Harvard Law School professor and activist Lawrence Lessig, has spent $1.4 million on mailers and television and radio ads supporting former Republican state Sen. Jim Rubens.
Rubens also enjoyed the support of the New Hampshire PAC to Save America, which spent $101,000 on direct mail and billboard ads before it closed up shop in August.
If he wins Tuesday, Brown will most likely welcome the continuing support of outside groups. While polls show him within striking distance of Shaheen, he'll enter the general election campaign with a cash disadvantage: in the most recent filings with the Federal Election Commission, Brown reported $1.2 million in cash, compared to Shaheen's $4.3 million.
