Yes we can retaliate, watch us, Tehran, Damascus, and I wouldn't rule out others.
OK, hypothetically we are talking about 20 Russian sourced nukes exploding in US cities.
I doubt that they will shoot their whole wad at once. They will go for maximum chaos. Blow one… make impossible demands. Blow another... make more demands.
We simply need something like the Russian "doomsday machine"
If the US is nuked, it starts a chain of events the leads to the extinction of the human race... Say we name a list of twenty possible retaliatory targets. Mecca, Pyongyang, Damascus, Tehran, Beijing, Moscow, Islamabad, Paris, Delhi, Jerusalem, Pretoria... on down the line
Simply put, we put all the words nuclear powers on a "hit list" Every explosion on our soil results in the random selection of an important city and it is obliterated.
...Every nuclear and terrorist nation "feels our pain"
A mass launch by any one (or all) nation(s) would be responded to "in kind."
Back to deterrence.
Time to put Mecca and Medina on the table. In other words ask: "What would Mohammed do? They'll understand, I'm sure.
One final thought: anybody vowing to diet, quit smoking, get sober, quit gambling, stop chasing women or procrastinating may want to postpone it a few months, you know, 'til after the election.