I do not think

that the so called "People's Pledge" will be honored for long in the current Massachusetts' senate race. We have too few restrains on the power of money in politics as it is with too much riding on the outcome to prevent an eventual abrogation of said agreement.
What enforces said Pledge?
Nothing, and we should count on nothing going forward.
And anyway, we as liberals and democrats are "doing business with religious sonsabitches"(expl. Karl Rove), and when that is the case you need to get it in writing, cuz their word ain't worth shit not with the Good Lord telling them how to screw us on a deal.

In a larger sense though, thanks to the Supreme Court's "Citzen's United" decision, the very Hobbesian war of all against all has been imported into US Elections. Money is a form of free speech so the SC sez.
Or from a purely Jeffersonian perspective, Money has the Right to Free Speech...and therefore a right to the franchise and therefore the right to organize the government if elections are dulty won in it's name.
Where does this all end?
Not in a good place I do think.
The Founding Fathers, almost to a man, feared the concentration of power as being fatal to the survival of liberty.
Thomas Jefferson, if he was alive today, would take one look at "Citizen's United" and immediately come to the conclusion that money is not a form of free speech, but is in fact, power in word & deed.
And power, is a thing that must be constrained in the Jeffersonian view.
So as this election goes forward, we need to look at the big picture...that is to say in a purely Jeffersonian context, are we now locked in a zero sum contest between the Power of Money and the "Rights of Man"?
Start there, tell me what you end up with.