However, groups did try to influence the race with mailers and organizational efforts, which were not covered by the pledge.
No other candidate has yet announced an intention to run for Kerry's seat, if he is confirmed as secretary of State, which is likely. But sources say Rep. Stephen Lynch (D-Mass.) is gearing up for a run and could launch his campaign as early as this week, creating an unwanted primary challenge for Markey.
Markey's proposal that all candidates, including any Democratic challengers, sign the pledge, could complicate Lynch's bid. Markey is a far more formidable fundraiser than Lynch, and without outside help, Lynch will have a tough time contending with Markey's campaign coffers.
However, Lynch's spokesman, Scott Ferson, said that the congressman had called for a people's pledge before and would sign it if he does enter the race.
"We're glad that Congressman Markey joins Congressman Lynch in calling for that," he told The Hill.
And it's unclear whether the likeliest Republican challenger, Brown, would agree to sign on to such a pledge. Some Republicans in Massachusetts feel that the pledge hurt the Republican in a year when Republican super-PACs played a large role in a number of Senate races nationwide, and that it handicapped Brown, who was out-raised by Warren throughout his campaign.
--This post was updated at 2:45 p.m. to reflect Lynch's position.