An abbreviated special election, however, would have a completely different set of circumstances than the Brown-Warren race.
The most significant impediment to a pledge banning outside ads would be a crowded Democratic primary. Assuming Brown runs as the only major Republican, it would be difficult to sign an agreement with several Democrats.
And even in the unlikely event that the Democratic candidates signed on with Brown, figuring out which candidate would have to pay penalties in the event of a violation would be a logistical nightmare. If an anti-Brown ad runs, would the Democratic field split the penalty evenly? If one of three or four Democrats is attacked, would the other Democrats share the penalty with Brown?
In addition, Democrats would be so busy fighting one another that they might be reluctant to sign an agreement with Brown that could hamper their ability later in the race, particularly if they are forced to spend heavily in a primary.
“Whoever comes out of the primary in a short election isn’t going to have the same type of name recognition and favorability” that Brown has, said Richard R. Tisei, a Brown ally and former Republican state senator who lost a bid for Congress in November.
For his part, Brown might be reluctant to wait until a general election campaign lasting only five or six weeks to sign a pledge, because it could force him to absorb attacks during the primary, without the ability to fight back against a clear opponent until the general election.
The other scenario involves Markey facing Brown without a serious challenger in the primary. In that case, a pledge is more plausible, though far from guaranteed.
Though Markey has some $3 million in his campaign account, he has yet to prove he can raise money at the same level that Warren and Brown did in the last election.
Brown may also calculate that he would be better off forgoing a pledge, so that outside GOP groups would be allowed to attack Markey, who is still largely unknown outside of his district. These groups could also help Brown compensate for the advantage that Democrats have in organizing voters.
“You want to define your opponent before they define themselves,” said Rick Tyler, a former adviser to Newt Gingrich’s super PAC, quoting a political axiom.
But even as political veterans believe the outside ads are unlikely to go away, they say they are sympathetic to the voters who will be barraged.
“There’s a still a lot of election fatigue,” Tisei said.
Voters, he added, would probably appreciate a break.