What is the nothernmost latitude in the US from which the entire southern cross is visible in degrees N?

The southernmost prominent star in Crux is Alpha Crucis, which is at declination -63 degrees 5 minutes (or thereabouts). The northernmost latitude at which Alpha Crucis can be seen is therefore slightly south of 27 degrees north.

The area assigned to Crux extends further south than that, but I presume you're interested mainly in the asterism (the "Cross"). At 27N, Acrux will fall just short of clearing the horizon at midnight (by about a fifth the diameter of the full moon); you'd need to be at about 26 degrees 50 minutes or so to ever see it rise above the horizon.

The only states from which it's ever possible to see the entire Southern Cross are therefore Hawaii, southern Florida, and the southernmost tip of Texas.