Iran doesn't need to cause mass casualties. They just need to make US citizens hate the war in Iran.
You know, I said that as a joke, but...
It's getting hard to tell jokes from reality with this administration (excluding roasts at the WHCD).
You would have to fire your entire nuclear arsenal and commit mass genocide.
Even then, you may not be able to destroy the missile cities, many of them were built to be nuclear resistant.
Look at what 9/11 did - for quite a while after the attack, which was heinous but in the grand scheme of things nothing compared to the scale of destruction that was visited upon Iraq and Afghanistan afterwards; And at least for a few years, very many Americans were in favour of those conflicts.