The easiest way to get a large nuke into and detonate in any of our cities would be to simply have it aboard a chartered private jet and detonate it before landing. I do not think that would be that hard to do.
"The Last Jihad" by Joel Rosenberg opens with an assault on the U.S. President's entourage at Denver International Airport. The attacking aircraft is a private jet inbound from Canada. It's not a big stretch to imagine a private jet in Canada as a platform to make a run on any major city in the northeast or midwest. Smuggling the weapon into Canada via ship or barge isn't that tough either.