"This is a great victory for the people of Massachusetts and a bold statement that puts super PACs and other third parties on notice that their interference in this race will not be tolerated,'' Brown said in a statement.

Republican Scott Brown and Democrat Elizabeth Warren signed a ban yesterday on third-party ads in their hotly contested US Senate race, after both sides postured to gain maximum public relations advantage from the agreement.

In her own statement, Warren heralded the achievement reflected by the agreement.

"With our joint agreement, we have now moved beyond talk to real action to stop advertising from third-party groups,'' she said. "But both campaigns will need to remain vigilant to ensure that outside groups do not try to circumvent what is an historic agreement.''


Brown, who has the most to gain from the agreement after already being outspent 3-to-1 by pro-Warren groups, immediately targeted online ads being paid for by RethinkBrown. It is a pro-Warren super PAC run by former aides to Governor Deval Patrick, a Democrat.

"We will suspend our advertising campaign, provided other third parties do the same,'' said RethinkBrown's spokesman, Steve Crawford. Other outside groups also promised to comply, with similar reservations.

The League of Conservation Voters, which has aired over $2 million in ads attacking Brown's record, said it, too, would try to honor the accord. The Massachusetts Democratic Party also said it would honor the ban.

Yet the challenge in enforcing the agreement, which echoed a failed 1996 pact between candidates William F. Weld and John Kerry, was immediately evident.

Crossroads GPS, a pro-Brown group, was noncommittal. "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 though,'' said Steven Law, chief executive.


Both campaigns acknowledged that broadcasters, who reap millions from campaign ads, are unlikely to turn down advertisements from outside groups.

Ed Piette, president and general manager of WBZ-TV and myTV38, said Friday that television stations have the right to reject issue ads and political action committee ads. Although he had yet to review the campaigns' pending request, he does not expect to turn down ads.

"We're a broadcaster,'' he said. "We're not in politics.''

Brown and Warren are the likely candidates in what could be one of this year's most hotly contested US Senate races.

Brown is battling to retain the seat he won in a January 2010 special election to replace the late Senator Edward M. Kennedy, while Warren, a Harvard Law School professor, is expected to emerge from a group of lesser-known Democrats aiming to win the seat back for their party.

Democrats currently hold a 53 to 47 majority in the Senate, but during this year's campaign, they are defending 23 seats. Republicans are defending only 10.

A net gain of four Senate seats would allow Republicans to take control of the chamber.

The third-party ad ban is designed to control what is already prodigious outside spending on the race. By some projections, the campaign could cost at least $60 million, with $20 million apiece being spent by the Brown and Warren campaigns and another $20 million being spent by special interest groups.

The outside spending is the most troublesome, the campaigns say, because it can be leveled by so-called 527 or super PAC groups that are funded by sometimes anonymous and deep-pocketed donors.


Glen Johnson can be reached at johnson@globe.com. Follow him on Twitter @globeglen.