You'd also have to ask the member states to give up their sovereignty. This wasn't easy even in the case of the US as there were a ton of issues that needed resolving (i.e. balancing power between small and large states.)
This would be incredibly more difficult in the case of Europe since the individual member states have had their own identity often going back two or even three millennia, not only that but what cultural identity would they take? I.e. little things like what common language will they speak? (Granted the US has no official language, but 80% of the population speaks the same one...such is by far not the case in the EU.) Also, I'm having a hard time seeing how e.g. England would agree to it, seeing as they even refuse to adopt the Euro (which it turns out was actually a good idea and worked quite well in their favor) and they don't even drive on the same side of the road as everybody else.
There is a large group of nations within the EU that have little problem with increased integration, Britain is in something of a small minority in its anti-EU stance. Until now keeping Britain in the EU has been seen as important and nobody really thought they should leave. Recently, however, the idea has been voiced in other EU countries that the British should just should just bloody leave if they have that stink in their nose rather continue this constant dithering. People are just getting sick of hearing Britain threaten to leave and then never doing anything about it, especially since it usually seems to be a smokescreen to extort special treatment. There is a whole bunch of things that can be done in terms of restructuring the EU if the UK is no longer there fucking things up to get special deals for it's financial industry. If the UK decides to go it will certainly be watched with great interest as they leave the common market, refuses to join the EEZ which is not an option for most of the UK Euro-skeptics/isolationists since it would involve enacting all those hated EU laws without any say in how they are made (a say which the UK currently has as an EU member). Ukip in Britain, the Freedom party in the Netherlands and Front National in France all believe that Europe is better off as a bag of squabbling nation states that Europe was before the EU was set up. The kind of squabbling, feuding bag full of angry weasels that would not have been able to agree on whether or not the Soviet Union was a threat for long enough to even conceive of forming an alliance against the Soviets to prevent them from gobbling Europe up one squabbling state at a time. NATO was only formed as a counterweight against the Soviets after several swift ass-kicks from the Americans and they cannot be counted on to the play the role of the big bad parent forever. So who is right? Is it Ukip and Co. who think they can take Europe back to being a bag of small squabbling nations and still be taken seriously by great powers like China, India, Russia and the USA? Or is it the so called 'federalists' who see increases in political and economic union as the only way to stand up to the big boys? You tell me? Which is more likely to succeed in helping Europe to deal with the Great powers of the 21st century? One big European cat or a group of cute little house-cats? If this reminds you Americans of a debate that took place in the US before the civil war about the pros and cons of increasing Federalism that is no coincidence. The one difference is that I am not nearly as alarmed at the prospect of a European civil war as some of the more delusional Euro skeptic wing nuts who seem to consider a pan European civil war to be just around the corner.