“It’s the only game in town and it will be difficult to keep the groups out, and I think both campaigns are going to think they need the help,” said Dave Carney, a longtime Republican strategist from New Hampshire who served as President George H.W. Bush’s political director.

In other words, you may want to avoid watching television, listening to the radio, or surfing the Internet this spring and summer. That’s when the state would hold a special election to replace John F. Kerry, who is expected to be confirmed later this month as secretary of state.

With only one Senate election nationally, a host of political groups from around the country would be able to focus all their attention and cash in Massachusetts, as they did ahead of the 2010 special election to replace Edward M. Kennedy.

“There’s a potential for a lot more money and a lot more third parties to play in this race,” said Doug Rubin, who advised Warren’s campaign and has agreed to advise the state Democratic Party for the special election.

Brown and Warren made campaign history a year ago when they signed a pact, dubbed the People’s Pledge. They agreed that if an outside group ran an advertisement on television, radio, or online, the campaign that benefits must pay a penalty to charity.

To the surprise of many, those groups largely abided by the Brown-Warren pledge, even as the campaign turned negative. That led some, including Brown, to speculate whether it could be duplicated elsewhere to curb an explosion in spending by outside groups.

“What we did together was historic and it’s something that I’m very, very proud of, and I know she is too,” Brown said the day before the election in November. “It’s really a model, I think, for the rest of the country.”

Rubin, who helped negotiate the pact with Brown, said he does not expect the next race to feature a similar agreement, but he believes Brown could pay a political price if he runs again and declines to participate, given his prior public embrace of the pledge.

Brown advisers have denied that he was hurt by the pact, though some political consultants say it cost him an opportunity to bruise Warren early in the race, before she introduced herself to voters on her own terms.

A Brown aide would not say whether Brown would sponsor a pledge again, given that he has yet to make up his mind about running and does not yet know the other candidates in the race. Representative Edward J. Markey, the Malden Democrat who is the only major declared candidate, has also not indicated a position.

There were numerous factors in the Brown-Warren race that may be difficult to duplicate in a special election.

The major one was money. Both candidates raised enough to spend about $80 million combined, shattering state spending records and making the race among the most expensive congressional campaigns in US history.

Unlike other races around the country, where one or the other candidate had a significant money advantage, neither Brown nor Warren needed help from outside groups to run continuous television ads.

Both campaigns also felt they had much to lose with the involvement of outside groups, whose televised attacks tend to be more incendiary than those launched when a candidate has to put his or her name behind the message.

Before the pledge, liberal groups were spending money on television ads attacking Brown at roughly double the rate as those from conservative groups attacking Warren, and Brown saw the first significant drop in his poll numbers.

Warren was a star of the Democratic base and many liberal groups hoped to raise money for their own purposes by promising donors they could help Warren get elected.

Brown’s campaign also worried that when conservative groups spent money in the race attacking Warren, she could simply use those attacks to raise even more money.

Warren’s side had its own fears. They believed the conservative money would catch up to the spending by liberal groups, given that pro-Republican groups outspent pro-Democratic groups in other Senate races by a 3 to 1 ratio, and that many had declared Warren their top target.