Answer:
The nationwide effect would be catastrophic
Step-by-step explanation:
I actually did a presentation on this a few years ago, so I hope this helps.
If the Yellowstone Caldera were to erupt, it would spew volcanic ash (which is sharp like glass) all over North America. It would kill all living things without protection. Focusing more on the United States, it would damage buildings, smother crops, and shut down many crucial businesses and services (such as grocery stores, power plants, schools, airports, etc). The gigantic ash cloud produced from this eruption would be shot miles up into the sky, blocking out the sun, causing a sort of an apocalyptic ice age on the earth below. Food is going to be hard to find for next year or so because of the cold conditions and the fact that plants will find it hard to grow without energy from the sun.
This is just a few of the nationwide effects. Don't even get me started on worldwide effects lol. I hope this helps!