Each of us is producer and also a consumer. However, we are much more specialized and devote a much larger fraction of our attentions to our activity as a producer than as a consumer. We consume literally thousands if not millions of items. The result is that people in the same trade, like barbers or physicians, all have an intense interest in the specific problems of this trade and are willing to devote considerable energy to doing something about them...The groups that have a special interest...are concentrated groups to whom the issue makes a great deal of difference. The public interest is widely dispersed. In consequence...producer groups will invariably have a much stronger influence on legislative action and the powers that be than the widely spread consumer interest.--Milton Friedman, from Capitalism and Freedom, quoted in The Regulation of Medical Care: Is the Price Too High by John Goodman, pg 95.
This quote nicely explains the source of the incentive to organize and lobby as a special interest group. The problem of special interests arises because we have allowed them to gain customized favors through the political process. The solution is to eliminate all possibility of special treatment for any group or individual by consistently applying equality before the law.
This is the proper understanding of "promote the general welfare." If a law does not treat all parts of society equally, then is it not promoting the general welfare but instead is promoting a special welfare of some at the expense of others.
The consistent application principle of individual rights includes the corollary principles of equality before the law and the promotion of the general welfare. Properly understood and implemented, these principles will protect us from the natural tendency of organized special interests groups to obtain special favors or privileges at the expense of the rest. These principles guide us in forming laws so that there are no legal conflicts of interests between rational men.