Well, there is also the other half of the theory, that if manufacturing jobs are created in say Vietnam, they will have money to spend, and they will want to buy products, products that preferrably come from america. That is a long term approach to the thinking, that everything balances out. There are obvious and clear holes in this thinking and it contradicts the view both parties are espousing today.
There is a trend of thinking that believes that all white collar jobs require a better educated worker then manufacturing jobs. Sales requirements are simply and generally the same as manufactering. An Example would be somone who manufactered say office supplies, loses that job, and winds up working at staples now selling the very items that they used to make.
However, the hitch is that, that job might or might not pay as well, its an unknown variable. A huge part of the free market and free trade theory that is espoused by both parties, are that we are turning into a service oriented economy, where our people now provide servies and no longer manufacter. There are some very obvious dangers to changing wholesale from going from manufactering to service oriented in policy.
If the manufacturing jobs are in say Vietnam, then they will have products to sell and the jobs that allow them to buy them. Americans sitting around with no paycheck are neither producing products or able to purchase them.