If the thing is powerful enough to take out the breadbasket when it does blow, then the world is in big trouble. I imagine that the US would still be able to grow enough food, but the exports will probably vanish. Not sure how much it would impact crops in the southeast, but surely it would.
On the plus side, the dust will stop global warming, right?
This is doubtful. Back in the '90's when the Mississippi flooded the US had to Import certain types of Wheat because the Crop damage was pretty severe. That was just the areas near and around the Mississippi that were affected. Just imagine if everything East of Yellowstone suffers severe Crop Damage/Failure.
That said, the US probably has a couple years worth of Grains stored away, but that just delays the inevitable. It would likely be that clusters of People would be favored and they'd be located around makeshift Hydroponic and other similar Food production facilities which had access to Electricity. 10's of millions would likely be left out of the loop though as even a small glitch in Production could lead to starvation for those lucky enough to be selected to enter such a facility.
As I understand it though, a Super Volcano isn't just about a massive amount of Ash that blocks out the sun. It also is a massive blast that would send a wave through the Earths Crust as well wiping out everything for quite some distance. I'm not really sure how far that blast reaches, but Seattle might have a significant Earthquake as a result.
It probably wouldn't take long before the whole concept of Nationhood would be just a distant memory and there would be a return to City State. Resources would be very tight and the last thing on peoples minds would be supporting any organization beyond their Local level.