After a two-week long tug of war, Harvard Law School professor Elizabeth Warren and U.S. Senator Scott Brown failed to reach an agreement to ban third-party advertising in the race for U.S. Senate in a meeting Friday.

Representatives of the Warren campaign walked away from the half-hour long meeting dissatisfied with the terms offered by Brown aides, according to Warren spokesperson Alethea Harney.

Ultimately the two camps could not agree on two issues: how to monitor spending on internet ads, and how to deal with so-called “sham” advertisements that masquerade as endorsements.

“The proposal he has put forward still includes loopholes that Karl Rove could drive a tank through,” Warren said in a statement, referring to American Crossroads, the conservative “super PAC” led by former presidential advisor Karl Rove.

Since December, Crossroads GPS, an independent political organization affiliated with American Crossroads, has sponsored two advertisements attacking Warren.

After the unsuccessful meeting, the Brown campaign issued its own statement expressing frustration and alleging that Warren is reluctant to move beyond rhetoric to achieve real action.

“She claims that she wants to move beyond rhetoric, but so far she has offered nothing but talk and excuses,” Brown campaign manager Jim Barnett wrote.

Prior to Friday’s meeting, Harvard professors called the efforts on both sides to reach a “truce” little more than political posturing.

“The public does not like [super PACs],” Harvard Kennedy School Lecturer David C. King said before the meeting. “So it’s easy for the campaigns to distance from them. This is complete nonsense. It’s all about trying to shape the impression of what is happening.”

Even if the two candidates had reached an agreement, the laws surrounding third-party organizations would not have allowed for any real enforcement, professors said.

Political action committees, or “PACs,” as they are known, enable donors to bypass campaign finance restrictions due to a recent Supreme Court ruling.

In the 2010 Citizens United case, the Court ruled that the government cannot restrict the political spending of independent organizations.

Though the organizations vary in scope and purpose, the 2010 Citizens United case has given rise to huge, money-rich political organizations like American Crossroads that have attracted attention nationally and in Massachusetts.

In her statement, Warren said she is still confident both camps can reach an agreement. No plans have been made for another meeting Harney said, but she added that the campaigns hope to continue a dialogue to iron out the loopholes.

Though the election is months away, Warren and Brown have been engaged in a fierce fight for the U.S. Senate seat currently occupied by Brown since Warren announced her candidacy in September.

—Staff writer Nicholas P. Fandos can be reached at nicholas.fandos@college.harvard.edu.