Sulfur,
I sympathize with your concern (especially as I am the #1 opponent of DC statehood without Guam statehood
) but Puerto Rico would probably just add a battleground state, not necessarily a Democratic state.
In fact, Puerto Rico is responsible for adding a new Republican to Congress - if results hold up, Luis Fortuno (PNP) will be the resident commissioner for Puerto Rico and he will certainly join the Republican caucus. (He's the former Republican national committeeman for Puerto Rico) He can't cast a vote, but he can debate and stuff.
http://www.luisfortuno.net/