It does not need to be a state for it to be part of the United States of America. Lots of places aren't states but are still part of the US. The federal district for instance.
You'll look pretty silly arguing that the National Capital is not part of the nation.
If you've visited the District of Columbia and walked its original perimeters, you can find the boundary stones set in place once every mile. Look along Western Avenue, Eastern Avenue, and Southern Avenue, where the District of Columbia borders the State of Maryland. I know that along Eastern Avenue, you can still find stones with "Maryland" carved on one side, and "Jurisdiction of the United States" on the other. The stones also had the date "1791" carved on them.
http://www.boundarystones.org/
"In 1791 and 1792, Andrew Ellicott and his surveying team placed 40 boundary stones around the perimeter of the District of Columbia, one at each mile of the original diamond shape. They laid the first stone, the south corner stone, at Jones Point on April 15, 1791, under the guidance of Benjamin Banneker. According to legend, "Banneker fixed the position of the first stone by lying on his back to find the exact starting point for the survey ... and plotting six stars as they crossed his spot at a particular time of night."
"From there, Ellicott's team embarked on a 40-mile journey that took nearly two years. They created the boundary lines of the capital by clearing 20 feet of land on each side of the boundary and setting a uniquely marked stone at each mile interval. On each stone, the side facing the District of Columbia displayed the inscription "Jurisdiction of the United States" and a mile number. The opposite side said either "Virginia" or "Maryland," as appropriate. The third and fourth sides displayed the year in which the stone was placed (1791 for the 14 Virginia stones and 1792 for the 26 Maryland stones) and the magnetic compass variance at that place. The stones along the northwest Maryland boundary line also displayed the number of miles they fell from NW4, the first stone placed in Maryland. Stones placed at intervals of more than a mile included that extra distance measured in poles. "
Maybe. Maybe not. That is a very tough legal question. Take a gander at the precedents of the Insular Cases. If the American legal system has a twilight zone, it's in Puerto Rico! Unfortunately, The SCOTUS is an appeals court and cannot just favor us with learned opinions on this, and the eligibility of fellow rumored to be a dual citizen of the UK and US moving into government housing without proper documentation.... unless someone makes a case and gets it up to them with all the right seals and ribbons attached.
Meh....
District of Columbia is OWNED by the federal Government. Ergo - American Soil. Puerto Rico is OWNED by Puerto Rico. It is NOT United States Territory, it cannot vote or even have a say in a Constitutional Amendment.
Is it getting clearer now? A Puerto Rican can be a Judge if also a US Citizen, but they are NOT Natural Born Citizens of the United States and are therefore ineligible.