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During Massachusetts' 2012 U.S. Senate race, Republican incumbent Scott Brown and Democratic challenger Elizabeth Warren hammered out what would become known as the "people's pledge" - a deal designed to discourage attack ads funded by outside groups.
It's since been proposed by candidates - Republican and Democrat - in Senate races this year in Alaska, Kentucky and New Hampshire. Candidates running in Rhode Island's Democratic primary for governor have already signed a pledge, and it's been proposed in gubernatorial and attorney general races in Massachusetts.
As often as not, however, the demand for a pledge seems to be as much about tweaking an opponent and wielding a political attack as it is about trying to tamp down outside money.
In Kentucky, Democratic candidate Alison Lundergan Grimes has urged her opponent, Senate Republican leader Mitch McConnell, to agree to a pledge.
"I further call on you to sign a people's pledge to ask all outside groups to cease spending in the commonwealth and allow the campaigns to deliver their messages to Kentuckians unvarnished," Grimes said.
McConnell campaign spokeswoman Allison Moore said Grimes is trying to score political points and believes "it's OK to barter our First Amendment rights away if it improves their electoral prospects" by limiting ads from outside groups.
In Alaska, Republican candidate Dan Sullivan has called on incumbent Democratic U.S. Sen. Mark Begich to sign a similar pledge that Sullivan said was modeled after the Brown-Warren agreement.
Begich's campaign has dismissed the pledge as a political ploy, noting that unlike the 2012 Massachusetts Senate race, there have been no negotiations between the campaigns - the proposed pledge was just plunked down at a Begich campaign office. The campaign also accused Sullivan of hypocrisy, nothing that Sullivan supports the U.S. Supreme Court's 2010 Citizens United decision that lifted restrictions on independent spending by corporations and labor unions.
Sullivan has said the question of a people's pledge isn't about Citizens United but rather is about "how Mark Begich and I are going to conduct this race," Sullivan said.
In New Hampshire, one of the signers of the original people's pledge - Brown - has declined to agree to a similar deal as he tries to unseat Democratic incumbent U.S. Sen. Jeanne Shaheen, who has pressed for a pledge.
The group Common Cause, one of the strongest supporters of the pledge, has urged candidates in more than a dozen states to adopt it. The group says the pledge will help, in part, to blunt an expected flood of money from outside groups in the wake of the Citizens United decision.
One of the few pledges agreed to so far is in Rhode Island, where the three leading Democratic candidates for governor reached a voluntary agreement in April to reduce outside spending.
The pledge signed by Providence Mayor Angel Taveras, Treasurer Gina Raimondo and political newcomer Clay Pell calls for any candidate who benefits from an outside group's advertising to make a donation to charity in the amount of the ad buy. If two candidates benefit, each will donate half the cost.
In Massachusetts, the pledge has had a spotty history since the 2012 Senate campaign, where it was largely successful in blocking television, radio and Internet ads by outside groups.
In 2013, U.S. Senate hopefuls Edward Markey and Stephen Lynch, both Democrats, agreed to an even tougher pledge that also targeted mailings to voters' homes. But Markey, who won, failed to persuade Republican nominee Gabriel Gomez to agree to the pledge.
This year, outside money has already begun flowing into the state.
In Massachusetts' contested 6th Congressional District, the conservative U.S. Chamber of Commerce spent $350,000 on ads in May to help support former state Sen. Richard Tisei, who is hoping to defeat Democratic incumbent Rep. John Tierney.
Associated Press writers Becky Bohrer in Juneau, Alaska, and Erika Niedowski in Providence, Rhode Island, contributed to this report.