LOWELL, Mass. — When Republican Sen. Scott Brown proposed a pledge to eliminate third-party ads in his race with Democrat Elizabeth Warren, it had all the makings of a well-crafted but wonkish piece of political theater. Press reviews would be positive, but its run would be short.

Surprisingly, 10 months and tens of millions of dollars later, the simple contract — imposing steep penalties on the candidates if the pledge was broken — has held. There have been many negative ads in this white-hot contest that could determine control of the chamber, but each begins or ends with the candidate standing by the spot.

The victor of the Senate race will be able to plausibly claim that the agreement — with its alliterative royal moniker, the People’s Pledge — was to his or her benefit. What is certain is that it irrevocably changed the contours of one of the most hard-fought races in the country.

How it did remains a matter of partisan perspective.

“Whoever wins will look back and say, ‘This helped,’” veteran Massachusetts Democratic strategist Mary Anne Marsh said. She explained that both candidates benefited in different ways. 

Marsh said the pledge gave Warren, especially early in the campaign, “some breathing room for people to get to know her on her own terms without third parties coming in here and defining her negatively.”

This year, Warren, a Harvard University professor and consumer advocate, was able to spend millions of dollars on positive biographical spots about herself before anyone disputed that narrative in paid TV ads.

But when the pledge was inked in early January, it was immediately advantageous to Brown. Before the agreement was signed, Brown was getting hammered by outside groups that were eroding his likability. Marsh, along with other Massachusetts Democratic and Republican strategists, said the cessation of those spots helped him stanch the bleeding.

“He was getting buried by these negative ads,” a Brown campaign aide said, pointing to polling that showed the negative ads, particularly from the League of Conservation Voters, had swayed public opinion against the Senator. “If that had continued, his image would have been ground to a pulp with all the outside spending.

“It’s hard enough to win as a Republican in Massachusetts. It’s impossible to win as a disliked Republican here,” the aide added. “Scott Brown would have been left for dead if those ads had kept running.”

Democrats insist, however, that Brown’s likability has taken a hit from having to stand by his attack ads on Warren. Without third-party groups to do much of the heavy negative lifting for him, he has had to risk his nice-guy image by saying he approves messages savaging Warren’s narrative.

“The fact that he has had to take the negative attacks against her and own them personally: That has hurt him,” Marsh said.

Republicans cry hooey, saying it is obviously Warren’s ads that have hurt his favorability with voters, not Brown saying he “approves this message” on his negative spots.

In interviews, both candidates framed the agreement as a greater good.

“Best thing I ever did,” Brown told Roll Call. “Best thing we ever did. And I give professor Warren a lot of credit for signing it with me and sticking to it. Because you would have had 20, 30 million — pick a number — of outside ads coming in,” he said. “So it’s a good thing for Massachusetts.”

Warren said  the pledge has “been to the benefit of the people of Massachusetts.” 

She pointed out that there was a difference between the campaigns’ contrast spots.

“I’ve been out there every single day talking about how Sen. Brown votes and talking about issues that matter to people here in Massachusetts,” Warren said. “Sen. Brown runs a different campaign,  a campaign of personal attacks. He’ll be judged by his campaign. I’ll be judged by mine.”

Pressed on whether he wouldn’t have been helped by some air cover from his allies, Brown said his opponent has “more allies than I do.” 

Maybe. Democrats make the opposite case.

“It’s been an advantage to have outside groups for every Republican Senate candidate in the country this cycle,” a national Democratic strategist said. “It’s naive to think it wouldn’t have been for him.”

The ban on outside spending was certainly helpful for both campaigns in maintaining their own message on the airwaves. With no independent expenditure groups, Brown and Warren could cleanly choose their own lines of attack. And Brown’s campaign avoided the residual negatives of having Crossroads GPS, a GOP group affiliated with Karl Rove, putting up ads in his favor in a state where most voters don’t think very highly of Rove.

Beyond that, it’s impossible to know exactly how outside spending would have changed this race. 

Insiders of both parties across the Bay State don’t expect to have to wonder in future statewide contests.

“I think these agreements are born of circumstances unique to particular races,” Massachusetts Republican consultant Rob Gray said. “I doubt we’ll see it again.”

Jason Kauppi, a Bay State Republican consultant, said the agreement only worked in this cycle in this state because the candidates appeared to be about evenly matched financially.

“It’s not going to have a long-term effect,” Boston-based Democratic consultant Scott Ferson said of the pledge.

In interviews with voters around the state, no one had particularly strong feelings about the pledge. But they did personalize the ads to the candidate, often saying they didn’t know whether to believe “Scott” or “Elizabeth.”

Polls continue to show a close contest between the two, but the dynamics of a Senate election in a presidential year in a state that President Barack Obama will comfortably carry inherently favor Warren. And while the race tilts in her direction, the fact that the incumbent has kept it close is a testament to his solid campaign, his excellent retail politicking and his likability in places such as Lowell, the largest city he won in the 2010 special election. It is a primarily blue-collar old mill city of about 100,000 people.

 In 2008, Lowell voted by a strong margin for Obama, but in January 2010, the city went 52 percent for Brown, then a little-known state Senator.

On Sunday, not long before the New England Patriots football game, a few patrons were watching TV here at Worthen House Cafe. 

Tim, who asked that his last name not be used, sat sipping a beer. Like most voters in the Commonwealth, he’s unenrolled with either party and plans on voting for Obama in two weeks. And, like most voters, he likes Brown personally, but he’s still undecided on the Senate race. 

He sees all the “mudslinging” ads on TV, which he compared to a nasty divorce. “‘You did that! ‘No, I didn’t, but you did this!’” 

He’s not sure who to believe about Brown’s claims in attack ads about Warren.

Tim explained that he doesn’t particularly care for politics and would make up his mind Nov. 6 when he gets to the voting booth.

Whoever he and similarly undecided voters cast their ballot for will win this race. And the People’s Pledge will play a part in the loser’s political post-mortem.

“If we win, everybody will say it was genius,” the Brown campaign aide said. “If we lose everyone will point the finger at it.”