Step 1. Target the 1950s era US ICBM fleet in their static bunkers. This only requires making the POTUS hesitate for ~7 minutes to "use or lose" them.
Step 2. Destroy the few dozen 1950s era upgraded B-52s and a few misc aircraft. Some might slip through but maybe not.
Step 3. Take out the ~14 outdated US ballistic missile subs using new supersonic anti-ship missiles.
Hopefully the US military will be able to upgrade the nuclear triad in time to maintain MAD and eliminate this potential threat. But I think the best solution would be for the entire G7 to develop independent MAD systems so there's never any doubt that it remains in effect.