A nation that has atom bombs could always simply use a bog-standard conventional submarine which could surface one dark night off the eastern or western seaboard of the USA, deposit a device on the coast and set it to explode an hour later. That would cause devastating damage - physical and psychological.
The only deterrent is the knowledge that the US could launch a massive response aimed at Chinese cities that would put China back to the Dark Ages. Unless the Chinese leaders are completely insane they would never risk throwing away the dominant economic position they've painstakingly established over recent years. The scary thing is that nationalism can be a species of insanity. And the one-child rule - combined with female infanticide - has given China a large surplus of testosterone-driven single males just itching for a fight to prove their manhood.