Either the president or people down the chain of command would hesitate or decide not to respond. And either way the result would be the same. Our total annihilation with no response.
At nuclear attack of size would wipe out a large part of humanity. Image dozens if not hundreds of Fukushima/Chernobyls occurring all at the same time. Until failsafe nuclear reactors are engineered and deployed the world can not afford a large scale nuclear exchange. Now a small exchange in an undeveloped area like the deserts of the middle east or the open waters of the Pacific is much more probable.