It’s a decision that might have cost former Sen. Scott Brown re-election and one Republican U.S. Senate candidate Gabriel Gomez has no plans to repeat.

At issue: the so-called “People’s Pledge,” where candidates discourage political ads from outside groups.

In 2012, Brown and challenger Elizabeth Warren signed a People’s Pledge. Many were skeptical that this pact would hold up, given the high national stakes. But it did, according to a new study by Common Cause Massachusetts.

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Pam Wilmot, the director of Common Cause says the study shows the People’s pledge was incredibly effective.

“The People’s Pledge significantly reduced outside money, it significantly reduced outside advertising, it limited dark or secret money in our elections and it boosted the role of small donors,” Wilmot said.

The study compared the 2012 Senate race in Massachusetts to three other competitive 2012 Senate races in Virginia, Ohio and Wisconsin. The study found, in the other races, upwards of 80 percent of all TV ads were negative, as opposed to Massachusetts, where 36 percent of the ads were negative.

Tyler Creighton, the author of the study, says the people’s pledge also reduced spending by outside groups.

“While all these races had $80 million-plus spent in them, in Virginia, Wisconsin and Ohio, over 50 percent of that money was spent by these outside groups that aren’t subject to disclosure requirements at all times, aren’t subject to strict contribution limits that allows wealthy donors to really dominate the dialogue. In Massachusetts only about 9 percent of the total money was from these outside groups.”

Just seconds after Ed Markey won the Democratic nomination, he hammered Gomez to sign the people’s pledge.

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“I want him to sign the pledge, the same way Scott Brown signed the pledge with Elizabeth Warren,” Markey said.

But Gomez says he absolutely will not sign it.

“I’ve been clear from day one,” he said. “I will only take one pledge, and it’s the pledge I took when I was a naval officer to protect and defend the Constitution of the United States.”

Tufts Political Science Professor Jeff Berry says it’s not to Gomez’s advantage to sign the pledge.

“He is quite a bit behind Markey in his capacity to raise money on his own, the race is very short, and he needs large amounts of money very quickly,” Berry said. “And outside groups can spend unlimited amounts, whereas individual contributions that come to him are restricted in amount.”

Markey has a lot more money than Gomez Does at this point. As of April 10, Markey had $4.6 million in cash on hand, and Gomez had $500,000.

But the question is, will the national Republican players decide to invest in Gomez’s candidacy?

Wilmot says if they do, and the candidates don’t sign the pledge, voters should beware.

“Big donors are going to be putting a lot more money in our elections, and that means they’re going to have a lot more chits to call in once one of those candidates is elected,” she said.

Wilmot says that’s the big problem. It isn’t just the election, but what happens after the election. And the people’s pledge, she says, is the best way right now to limit big money in politics.