Two Democrats battling to win the Democratic nomination to succeed Sen. John KerryJohn Forbes KerryFor the sake of national security, Trump must honor the Iran deal Bernie Sanders’s 1960s worldview makes bad foreign policy DiCaprio: History will ‘vilify’ Trump for not fighting climate change MORE (Mass.) have agreed to restrict outside spending from super-PACs and advocacy groups in their primary.

Reps. Edward MarkeyEdward (Ed) John MarkeyDems lambaste Trump’s ‘outrageous’ EPA chemical safety pick Dems plan to make gun control an issue in Nevada Dem senator pitches ideas for gun control after shooting MORE and Stephen Lynch modeled their pact after the deal between Democratic Sen. Elizabeth WarrenElizabeth Ann WarrenOvernight Finance: Lawmakers grill Equifax chief over hack | Wells Fargo CEO defends bank's progress | Trump jokes Puerto Rico threw budget 'out of whack' | Mortgage tax fight tests industry clout Michelle Obama is exactly who the Democrats need to win big in 2020 Wells Fargo chief defends bank's progress in tense Senate hearing MORE and Republican Scott Brown in last year’s Senate race.

Under that agreement, a candidate who is the benefactor of outside advertising will have to pay 50 percent of the cost of airing the ad to a charity of the opponent’s choice.

But the new pact goes further than Brown and Warren by restricting direct mail from outside groups ­— something left out of last cycle’s pledge — as well as TV, radio and online advertising.

It is unclear if Republican candidates in the race will sign on to the pledge in the general election, where outside spending will have its greatest impact.

Gabriel Gomez, a local businessman and former Navy SEAL, told the Boston Globe he had not made a decision on the pledge. He entered the race earlier this week. 

The pact, dubbed the People’s Pledge, didn’t catch on in other Senate races last year, though a number of Democratic candidates did propose similar ideas. The success in Massachusetts seemed to hinge on Brown, a centrist Republican who supports campaign finance reform.

Brown and Warren’s pledge last year held up for the most part. In one of the most expensive Senate races in the country, outside spending was only about one tenth of what the two campaigns spent. Outside spending totaled about $8 million compared to $77 million spent by Brown and Warren.