Though today's Democratic primary in Rhode Island's governor's race might not generate much national press attention, the race is significant as a case study of a way to stop the flood of unregulated outside money in the post-Citizens United era.

Because of the Supreme Court's 2010 ruling in Citizens United v. Federal Election Commission and subsequent court rulings, super PACs and 501(c) nonprofit groups may accept unlimited contributions from corporations, unions, and individuals and spend them to influence elections. Congress and state and local jurisdictions have no authority to block now largely unregulated spending by outside groups. Unless the Supreme Court overturns Citizens United or the nation passes a constitutional amendment permitting Congress to set reasonable limits on campaign contributions, candidates may be the only bulwark against outsized outside spending.

The Rhode Island race marks the second time that candidates have mutually agreed to work together to stem the tide of outside money in their races.

The first time was in the 2012 Senate race, when then-U.S. Sen. Scott Brown (R-Mass.) and challenger Elizabeth Warren (D) each pledged to donate 50 cents to charity for every dollar spent by outside groups to assist their campaigns. The agreement became known as the "People's Pledge," and outside groups mostly honored the candidates' wishes. The agreement was violated twice, both times by groups supporting Brown, who paid the financial penalty.

In April 2014, the three Democratic candidates for governor of Rhode Island reached a similar agreement that was facilitated, monitored, and enforced by Common Cause Rhode Island. The impetus for the pledge in Rhode Island was a $100,000 contribution by a Houston, Texas, couple to American LeadHERship, a super PAC that had been formed to support one of the candidates for the governor. In an effort to prevent this type of spending from taking over the race, a second candidate proposed an agreement similar to that used by Brown and Warren, and the other candidates agreed.

The Rhode Island pledge discourages spending by outside groups on television, radio, and paid internet advertisements. If a group violates the pledge, the candidate whom the expenditure was intended to benefit will donate 50 percent of the cost of the expenditure to charity. The agreement does not prohibit outside groups from spending money on canvassing, direct mail, or other forms of unpaid media.

Although some outside groups have paid for direct mail and internet videos, the pledge appears to have been violated only once. On September 4 and 5, the Providence Fraternal Order of Police purchased a $1,200 advertisement in the Providence Journal attacking Taveras. This was a tiny expenditure relative to the candidates' combined spending of $12 million, which appears to be a record for a Rhode Island gubernatorial primary.

After it was determined that the advertisement constituted a violation of the pledge, Pell and Raimondo both agreed to pay the penalty.

In retrospect, it's not surprising that these types of pledges would discourage outside spending. Besides the financial penalty the candidates would pay if outside expenditures are made to assist them, candidates would also be viewed as "guilty by association" if an outside group violated the pledge, opening the door to voter reprisal against the candidate the outside group intended to help.

Candidates in Alaska, Rhode Island, Massachusetts, Kentucky, and New Hampshire have proposed the pledge, but their opponents have not accepted. In order to combat the corrosive impact of outside spending, more candidates need to agree to say no to outside spenders and yes to voters.