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.”