The U.S. could not mobilize today as it did in WW2.
First, I don't think our existing industries could be as easily converted to building modern weapons. I believe the gap between what is required to build an Abrams tank and a modern automobile is rather greater than between what was required to build an M3 halftrack or Sherman tank vs a 1940 Ford car. And the existing factories for military production are capable of producing only rather small quantities of systems; it's not like they have huge "slack" capacity. So new factories for producing tanks and other heavy vehicles will need to be built from the ground up. I don't know the situation with advanced airframes and high-performance aircraft engines, but I suspect some of the same factors apply.
We can't do it with personnel, either. First, attitudes toward the government and acceptance of how much control it should be allowed over the lives and choices of individual citizens has changed. The draft will be resisted to a much greater extent than in 1941. People question the truth of what the government tells them more nowadays, and i think they are far more likely to simply say "Why should I fight in a war that was only made necessary by meddling inappropriately and mismanaging foreign policy? Our own country is to blame for this crisis, and I won't support it."
The level of provocation needed to cause 1941-level unity of purpose and acceptance of hardship is likely to be even greater than that needed to cause leaders to order mobilization in the first place.
American youths are, on average, much more sedentary that in the early 1940's. This means that a very large proportion are unfit for service. It's not as simple as saying "We'll just kick their butts harder". Even today's softened-down Basic breaks a lot of recruits. Some wash out during Basic due to physical injuries or conditions caused by the strains of getting fit; many others are pushed on to AIT broken, where the effort to physically rehabilitate them seriously detracts from training, and all too often fails. It was not uncommon for me to see 20-plus "profiles" trailing along behind a 200-strong formation marching to PT. I know for a fact that many were faking, but this is revealing, too: if a fair fraction of the minority of American youths who were motivated enough to even try joining the Army, are then willing to fake injury in order to quit after just 3-4 months' service, you've got a problem. What will happen among draftees who didn't want to be there in the first place, and whose beginning fitness level is even lower on average? Then consider how all Americans -- but particularly parents -- are not as accepting of truly brutal training conditions, so instituting a really tough Basic will be difficult in the first place -- and really tough Basic is what you'll need.
On the subject of ADD/ADHD, many parents are going to raise holy hell if the incorrect diagnosis for these conditions gets overturned by the military; expect lots of complaints and pleas for help to Congressmen. In the case of legitimate diagnoses, yes these can often be managed through proper diet and drugs. But the logistical effort to provide this diet and these drugs will be an additional burden during wartime, and is guaranteed to often fall short, which will have negative consequences if large numbers of ADD/ADHD people are in service.
One last note on fitness. Although it is true that the physical demands of being a support troop are much less than those of being a combat infantryman, they are still (particularly in a non-static combat theater) quite a lot higher than what sedentary American youth are accustomed to. So even when training to fill support roles, you'll have to encounter a high wastage rate in Basic training; you cant just throw couch potatoes into even these relatively less demanding positions.
Oh, and you'd better think twice before drafting females. Americans seem to assume that women can do absolutely anything that men can, and every bit as well*, and are determined that they get the chance -- when and if they so choose. But I don't think they are ready for women as a whole to be forced to accept the responsibilities implied by this. We'd draft fathers, but no way we'd do the same with mothers, for instance. It might be interesting to note that female soldiers (at least in the U.S. Military system) pose a considerably higher logistical burden during deployment to a non-static theater than males, due to health and hygiene requirements. Further, I have been in units where literally no less than 1/3 of female soldiers suddenly become pregnant in the months just before deployment. If you start drafting females, expect a largish and sustained baby boom in the civilian population.
These are just my thoughts, not the opinions of any U.S. service branch. And if they seem negative, they are based on 20 years in the Army, plus 45 years' observation of the American public (especially the rather profound changes in the conditions and attitudes of youth, just between the 1970's and today).
Edit: my opinion is that, considering industrial and social factors, the U.S. could conduct a WW2-level mobilization anytime during the 40's through the later 60's with somewhat increasing levels of difficulty. From the
late 60's through perhaps the 80's, its ability to do so becomes problematic. From the 90's until today, I think it is effectively impossible. The U.S. could still mobilize strongly, but not in the way ot to the level it did during WW2. At least, not unless the provocation was so traumatic that it caused a profound change in the most basic American mindset overnight, which I think is harder to achieve than simply convincing American leaders that a full mobilization is needed.
*I'm not about to enter a debate on differences between males and females, and what implications this might have for serving in various military capacities, so don't even try to draw me into such a conversation. I will say that I think females statistically perform as well as males in most roles, and perhaps statistically better in some roles. I completely support the idea of females serving in the military in general.
