We’ve already had many significant earthquakes and many significant hurricanes. That we will have some more is 100% probability. While really bad, those tend to be relatively localized. The undamaged rest of the country can pitch in to help.
We’ve already had a significant terrorist attack. The really bad future one would be multiple dirty bombs that would render a lot of cities uninhabitable.
A financial collapse would hit everyone.
Most popular Republican answers:
Global thermonuclear war
Most popular Democrat answers: