This comes up every four months, like someone wants the story to stay alive. It is a very unlikely attack scenario.
The attacker would need to not only make a 100% reliable thermonuke, he’d need to deliver it and have it detonate at just the right altitude over CONUS. Oh and it would need to be completely stealthy so AEGIS, THAAD, Standard Missile 3, BSTS, and GMD systems would not see it and zap it.
Then, if they manage all this, they will have hurt commercial and public systems, but would leave potent military systems still running and ready to backtrack the missile path and respond.
Far more likely that they’d smuggle in their warhead disguised as cocaine, put it in a Cessna, and fly it over New York (maybe over Yankee Stadium dragging a banner).