Post-war US had a period of 20-25 years where it was a colossus. As the rest of the world recovered, it was natural that US exports as a percentage of the world total would decline. And since the US has a relatively free market for imports, US manufacturers should have expected more competition.
(Recall that the US still is the largest manufacturer in the world.)
Women entering the workforce in ever larger numbers also played a role in driving down labor's relative strength, of course. But women were often entering the workforce out of necessity, so they were as much an effect as a cause of the relative drop in power of labor relative to management. (Not to mention the fact that everyone should have the opportunity to do what they want with their lives and not be force to assume a "traditional" role...)
While you could (I suppose) argue that regulations on (say) autos caused problems for GM, I think the case is very weak. Air bags and other safety requirements, pollution controls, and mileage requirements didn't keep GM's production from increasing along with their profits. In fact, I think that the case can be made that the regulations kept GM in business since they forced the company to improve their cars and allowed them to increase their prices in the process.
GM's, and too many other large corporation's, management was too conservative and too unwilling to take risks to change the company to meet new realities. They didn't like change, so were unprepared as the world changed around them.
I think the case is even weaker in many other industries: railroads, heavy equipment, industrial tools, and even in basic materials like steel. Too many of these industries were resting on their early-20th-century laurels and not investing in their plants and people.
Unions may have played a role in some specific instances but aren't the general cause. I don't think you can honestly look at the declining percentage of union membership over time and say that they somehow destroyed US competitiveness. If that were the primary cause, then Japanese and German auto industries would have been in the toilet as well rather than eating GM's lunch.
http://www.cepr.net/...acturing-in-2006/
Their management did them in, with the help of their "investment advisers" whose idea of improved processes was to buy up a competitor, combine management, fire productive employees, and muddle along for a few years until the IAs decide that the company should be broken up to components to make it more "nimble" - while the IAs collect fees all along the way... See, e.g., ITT, GM, AT&T, etc., etc.
But, I guess it's easier to blame the Gubmint... :-/
Cheers,
Scott.
(Who recognizes that if this topic were easy to argue then it wouldn't be controversial...)