My belief is that promotion is more broad than a marketing campaign (think the four p's). A marketing campaign is found within promotion. I was also assuming a campaign is a strategy of speaking to a particular segment within your target. Is this the case? What if, for example, you have marketing activities you are doing specifically for Christmas. Could this be the Christmas promotion within a wider campaign?

Marketing campaign involves advertising, hiring a staff of salesmen and women to go out and solicit business, and finding retailers who will carry your product on their shelves. Promotion is temporarily reducing the cost of your retail product (discount sale) to steal your competitors' customers, by trying to convince consumers to switch services or name brands. People are creatures of habit; they'll routinely buy the same brand of products or services, until a better value comes along to entice them to switch.