Remember there are ICBM, Theater Tactical missiles, submarine-launched ballistic missiles (SLBMs), and aircraft borne nuclear bombs and nuclear-armed Advanced Cruise Missiles.
That's a lot of variety in methods of deployment. I have no doubt that a few hundred would get through on both sides.
Those Russian nukes, for all we know, could already be here.
The southern border has been wide open; if millions of people and tons of drugs can enter the US, an enemy with nukes could have already brought them into the US.
Hiding them, storing them. Just waiting for the moment to park them next to their targets.