U.S. Sen. Scott Brown, R-Mass, and Elizabeth Warren, his main Democratic challenger, will have difficulty enforcing a pledge to limit third party spending in what is likely to be a closely watched and bruising Senate race.

Brown and Warren have agreed to donate to charity 50 percent of the cost of any television or radio advertising purchased by outside groups. The pact is an attempt to curb the influence of outside organizations that have begun pouring millions of dollars into political campaigns after the Supreme Court's Citizens United Decision eliminated some restrictions on campaign finance.

But the Massachusetts senate seat candidates face a challenge in upholding their commitment in an electoral landscape that has been fundamentally reshaped by an avalanche of money from entities known as super PACs. Candidates are barred by law from coordinating with super PACs, weakening Brown or Warren's ability to check their spending.

Outside Groups Can Still Spend

These are by definition of independent groups, said Adam Skaggs, a senior counsel at the Brennan Center for Justice's Democracy Program. Perhaps this will reduce to some extent the amount of advertising, but this is going to be a heated race and obviously this pledge is not binding on any of the outside groups in a position to run these ads. I would not be surprised as the race heats up if outside groups decide to jump in.

Such groups have already played an aggressive role in a Massachusetts Senate race shaping up to be one of the costliest in history, with Brown and Warren reaping a combined $9 million in the last three months of 2011. The League of Women Voters and the League of Conservation voters have spent about $2 million attacking Brown's record, while the powerful conservative group American Crossroads has spent more than $1 million on ads attempting to portray Warren as a radical allied with the Occupy Wall Street movement.

Plausible Deniability

While the agreement could bolster Warren and Brown's contentions that they are trying to run clean, civil campaigns, it is also likely reflects a calculated strategy to shield themselves from attacks.

Part of the motivation is to try and keep the thing from going totally into the gutter, and the other is smart politics, which is to keep them from having to defend themselves against brutal smears, said Tobe Berkovitz, an associate professor of communications at Boston University. At least if you have attack ads from your opponent you can hold your opponents responsible whereas with the outside groups there's plausible deniability.

Berkovitz noted that television stations could play a pivotal role in the dynamics of the race. Television stations do not have to accept advertisements funded by independent expenditures, so the candidates could put pressure on local stations to help enforce the pact.

Otherwise, it is unclear how the agreement will hold if outside groups continue to spend money. Warren alluded to that possibility on Friday, saying that Brown's proposal had loopholes Karl Rove could drive a tank through in a reference to the leading Republican operative who is associated with American Crossroads. American Crossroads leveled a similar criticism on Tuesday.

Because the agreement allows union phone banks, direct mail, and get-out-the-vote drives - all union core specialties - Warren's latest agreement has loopholes the Teamsters could drive a truck through, the longshoremen could steer a ship through, the machinists could fly a plane through and government unions could drive forklifts of paperwork through, American Crossroads CEO Steven Law said.

Super PAC spending has already played a powerful role in the Republican presidential race. An affluent Newt Gingrich backer said on Monday that he would pour $5 million into a super PAC supporting Gingrich, an attempt to match the clout of the pro-Mitt Romney Super PAC Restore Our Future. The same donor helped propel Gingrich to victory in South Carolina with a $5 million infusion that helped pay for advertisements attacking Romney.