We (the US) can build anything the Japanese do. In fact, we do already. So its not a question of technical capability.
Its not so much 'Buy an American car' as it is 'help bail out the S corporations'. The government is coming up against the realization that, if GM and Chrysler fail (make that 'when', not 'if'), they (or more accurately, we) will be on the hook for their workers pension and healthcare costs. If we can just keep them limping along until things pick up again, we can postpone a mass of people getting dumped onto the public dole. And since one of the current administration's goals is is to sell a publicly funded healthcare system to Congress, they need to minimize the shock of dumping a huge obligation on it from day one.
Eventually, such a system will make it easier for the gov't to kiss off failing corporations, since they won't have to deal with the political ill will of ex employees (voters) with no financial safety net. But we're not there yet. And GM is holding a gun to the heads of their hostage workforce, figuratively. 'Give us the money or these poor workers will get it'.