It's both the charging port AND the charger cable. The port gets sloppy and the tensioner clips on the charger get recessed over time.
Since it's not practical to fix the expansion and wear on the shell of the port the solution is tweaking out the clips on each side of the charger cable plug, about once a month.
Not a big deal. Better than returning your phone and getting someone else's abused phone which will likely see the same issue appear in time.
Edit: After tweaking on several chargers for a few months mine has gotten worse and now it is apparent to me there must be an intermittent loose connection between the USB charge port and the phone's board. I'm usually pretty good at taking things apart and fixing them but I know my limitations and this may exceed them. I made an external charger and will buy an extra battery. (just because VM's new phone offerings don't excite me or those that do are out of the budget now- and I'm still on the 25 a month plan)