It is a nice idea, however once you start to formulate a pledge it becomes quite a complex task. On the one hand the pledge needs to describe the outcome, preferably in a concrete and measurable way. On the the other hand policy advocacy is concerned to propose the actions that would lead to the outcome. So "cut unemployment by 500,000" would be an outcome, but what policies would lead to that result?
The challenge forces you to look for a symbolic policy which provides an example of a deeper idea. Given my simple Keynesian approach I want to argue for more investment, but to what concrete result? I have settled on this idea:
We will build 100,000 affordable homes, sell them and use the proceeds to build 100,000 more.It provides an example of a simple Keynesian policy. When the private sector is unwilling to invest the government can step in and undertake the investment itself. The resulting assets can then be sold to the private sector. It has the beauty that the initial borrowing can be repaid once the economy recovers simply by selling all the assets. In my pledge example, the government would borrow £3bn to build the houses which would be sold on the open market to households or housing associations, realising £3bn+ in revenue which can be used to continue constructing housing. Once the private sector begins to build again, the government scheme can be wound up and the money repaid.
I would advocate giving a government agency the responsibility of handling this fund. I would also back up the policy by creating compulsory purchase powers to allow the agency, or local government, to acquire any brownfield site which has had planning permission for housing for more than five years without the owner developing the site.