Viewers from Florida to Montana have been deluged for months by grainy images of politicians on Wild West wanted posters, cash changing hands in dark rooms, grandmothers losing benefits, and music that might be used to herald an impending shark attack.

Yet even as spending from super PACs and other interest groups reaches new levels this year, with more than $90 million spent in 16 states with Senate races, the money has not touched the Massachusetts Senate race since January.

A groundbreaking pact between Senator Scott Brown and Elizabeth Warren to prevent third-party ads has survived, to the amazement of partisans around the country who expected it to crumble in the midst of what may be the nation’s most competitive Senate race.


In January, Warren and Brown agreed to a voluntary enforcement system to keep interest groups from running ads aimed at influencing the election. If an outside group runs an advertisement on television, radio, or online, the campaign that benefits must pay a penalty to charity. For example, if an anti-Brown group spends $1 million disparaging him, Warren must donate $500,000 from her campaign account to the charity of Brown’s choice.

The agreement was tested early, when two groups spent relatively small sums on Brown’s behalf. Brown’s quick agreement in March to donate $1,000 and $34,545 checks to the Autism Consortium helped erase doubts that the candidates would comply.

“I paid money already,’’ Brown, a Republican said recently, underscoring his commitment. “I’m hopeful that it continues.’’

He called the pledge a mark of “two good people’’ who feel voters deserve a better campaign. “I’m going to continue to tell people not to get involved,’’ he added.

Warren, a Democrat, said in a statement that she’s proud as well. “The people of Massachusetts are entitled to hear from the candidates themselves,’’ she said. “That’s how elections ought to work.’’

The pledge’s durability has hardly meant kumbaya between the candidates, who continue to battle rhetorically on a near-daily basis. Even the terms of the agreement were a hard-fought media contest, with the Brown and Warren camps fighting over credit for initiating what Brown dubbed “the People’s Pledge.’’


But since the pledge took effect, the advertisements have been about the candidates’ own biographies or agendas, in large part because they have been forced to put their own names behind their messages. As a result, Brown and Warren have been unwilling to risk alienating voters with the kind of slash-and-burn ads routinely run by outside groups.

The lack of negative ads may have kept a bad campaign situation from becoming even worse for Warren, when her unsubtantiated claims of Cherokee heritage dominated the news for more than a month in the spring. No one has made an ad about it, at least for now.

“If a group was engaged in the race on the right [or] the center-right, they would have pulverized her, maybe taken her out,’’ said Dave Carney, a longtime Republican strategist from New Hampshire who served as President George H.W. Bush’s political director.

Brown has also been helped, though perhaps to a lesser extent. He has avoided commercials based on the attacks Democrats have made, linking him with Wall Street bankers and Representative Paul Ryan, the GOP vice presidential candidate, in an attempt to tarnish his independent image.

Massachusetts television viewers have not been completely spared from super PAC ads because New Hampshire, a swing state in the presidential election this year, shares a television market with Boston. But it has been nothing like Missouri, where a crowded Republican primary for Senate followed by a competitive general election have turned the airwaves upside down.


“It’s really hard at times to figure out exactly who was attacking who,’’ said Peverill Squire, a political scientist at the University of Missouri who studies elections.

Before the recent Missouri primary, there were a slew of ads, often with confusing agendas. In one, a Democratic group was running a spot that seemed to favor a GOP candidate, Todd Akin, who many Democrats believed would be weaker in the general election against the Democratic incumbent, Senator Claire McCaskill.

“It just sort of feeds into the general distemper of the times regarding politics,’’ Squire said. “It makes people who are unhappy even more so.’’

In Montana, ad time is relatively cheap and Senator Jon Tester, a Democrat, is in a tight race with US Representative Denny Rehberg, a Republican. Voters are also contemplating an open House seat and a race for governor.

As a result, the ads are nearly nonstop, said Tim Keating, regional and national sales director for a group that owns all five CBS affiliates in the state.

“They’ve all gone negative,’’ Keating said of the Super-PAC ads.

Keating said he takes a lot of ribbing at backyard barbecues, and has to explain to people that he merely sells the ads; he doesn’t make them.

“My defense is, you know, there’s only [so many] days left’’ until the election, he said.

The Massachusetts pact has survived not because the candidates like each other, but because both so far believe it is in their interest. It has not been replicated elsewhere because few other races are as evenly matched in polling and as well-financed in campaign dollars, political analysts say.

Warren and Brown are among the nation’s top money-raisers, with enough cash in the bank to fill the airwaves through November without outside help.

Warren’s side feels she had much to lose from outside attacks, especially early on, when she had not introduced herself to voters. Brown’s side feels he has much to lose, because numerous left-leaning groups around the country view Warren as their top electoral priority and were spending aggressively against Brown before the pledge was signed.

Nationally, pro-Republican groups have spent about three times more on Senate campaigns than pro-Democrat groups, according to a Democratic official in Washington. But in Massachusetts, the dynamic was different. Before the People’s Pledge, officials from the Democratic and Republican parties said that groups had spent about $3 million on ads against Brown versus more than $1.6 million against Warren.

The pact also serves the interest of the candidates’ political consultants, who stand to gain more control of the candidates’ messages by keeping the outside groups away.

“It’s very difficult to run a campaign when you have third-party groups out there advertising in a way they think they know best,’’ Carney said.

Political consultants also make much of their money from the ads they place on television. With all advertising now going through the campaigns, that translates into more money for their consultants.

“The big money is media buying,’’ said Rick Tyler, a former adviser to Newt Gingrich’s super PAC and sometime critic of the current system.

To that end, the pledge has also helped drive the campaigns’ coffers to near-record heights, because that’s the only way donors can help the candidates.

There remain lingering doubts that the pledge will endure. The race has remained tight for months, a statistical dead heat in most polls. But some predict that if one candidate begins to thrive, interest groups will come in to help the other.

Democrats point out that a pair of groups founded by Republican strategist Karl Rove, Crossroads GPS and American Crossroads, have never promised to abide by the pact, even though they have so far. A spokesman for Crossroads, Nate Hodson, said the group plans to spend a total of up to $70 million on Senate races nationally, but he otherwise declined to comment. Republicans say liberal groups are eager to buy ads for Warren, who has become a standard-bearer for the left, so they can use her election to help raise money for their groups.

“We’ll see what happens,’’ said Navin Nayak, senior vice president of campaigns for the League of Conservation Voters, which spent about $2 million attacking Brown before the pledge was initiated. “We have no intentions of breaking the pledge, but the floodgates may open up on the other side.’’

Like other groups, the US Chamber of Commerce, which endorsed Brown on Wednesday, has also not made any promises.

Rob Engstrom, the chamber’s political director, said the race remains important to his group, which spent about $1 million helping Brown in the 2010 special election, according to the Center for Responsive Politics. News reports said the group spent about $50 million nationally on congressional races in 2010, and Engstrom promises to spend “significantly more’’ this year. He said the Brown-Warren race is among 13 Senate races in which the chamber is active, all on behalf of Republican candidates.

“Make no mistake, we have been and will be aggressive in the Massachusetts Senate race to educate our members and the voters as to where these candidates stand on the issues,’’ he said.

Engstrom won’t say what tactics the group will employ, but he emphasized that the group has significantly increased its grass-roots efforts in recent years. He said the number of times its members contacted lawmakers went from 40,000 in 2008, to 1.3 million in 2010.

Liberal groups are also working other avenues, especially labor unions, which tend to have an advantage in organizing direct outreach efforts. But groups launched specifically to get involved in the race have been hampered without the ability to advertise.

“We continue to operate. We continue to publish online material about Scott Brown’s record,’’ said Stephen Crawford, spokesman for Rethink Brown, a union-backed group launched last year, which ran extensive Internet advertisements before the pledge. “We engage the news media when we can about aspects of his votes, current and past, that we think are pertinent in the race.’’