"It's hard to view Jeanne Shaheen's actions as anything other than hypocritical and self-serving," Brown said in a statement. "The people of New Hampshire can see through the Washington-style game she is playing."
Brown argued that Shaheen and pro-Democratic outside groups have already been attacking him.
"Before I even thought of becoming a candidate, Jeanne Shaheen's allies in Washington were running negative ads against me for months," Brown said. "And right now, while I'm meeting with the people of New Hampshire, she is on the West Coast raising money so third-party groups in DC will have money to run even more outside negative ads against me."
The pledge Shaheen proposed says that both candidates must pay for 50 percent of the cost of advertising run by outside groups supporting their preferred candidate, according to The Hill. That applies to online and broadcast ads and include both negative ads and positive ads.
The pledge definitely limited spending in the Massachusetts race between Brown and Warren. After they signed the pledge in 2012, the spending from outside groups was only one tenth of what either campaign spent in total. Outside groups spent about $8 million while both campaigns together spent $77 million, according to the Center for Responsive Politics.