They cannot. Once the transaction becomes part of the blockchain, its replicated to each and every node in the world (we are talking about thousands) that span the length and breadth of the planet. Thus, its practically impossible.

Quick thought experiment: I just did a quick back of the envelope calculation on the brute force approach. 50% of the current hash rate would be 548 million retail - (current rate 8M TH/s, 1850$ retail Antminer S9 rated 13.5 TH/s). That's a quick thought on naive approach - not sure I have that right, so please check my work. I have a few other ideas as well. edit: of course, the existing hash rate would still be there, so you'd have to double it, not buy half. I still think the retail estimate is on the high side (markup, not leveraging economies of scale). My guess is that an organization like DARPA could buy Bitcoin for <250M$.

I think large states definitely have the resources to launch 51% attacks

But large states cannot do it in a legit manner as they have to follow some constitutional rights of their citizens. Most constitutions of these countries have enough freedom rights paved into them to avoid such blatant action by the government. Besides, the backlash in the media would be unbelievable, the party that does this kind of thing may not find many voters in the next election due to the negative press (at least in technologically advanced nations like USA and Sweden where people are generally aware of online rights).

Which US constitution clause would prohibit the US government from launching a 51% attack?

The fourth amendment?

Where you live sounds very nice.