If conservatives weren't trying to be racist, they'd have let Puerto Rico in ages ago and picked up some more senators, which is why Democrats never seem particularly interested in pushing the issue.
I was always under the impression that the D's would pick up seats if PR became a state, that GOP assholery like Trump's treatment of PR after the hurricane, meant that it was a sure thing for Ds. But apparently it is WAY more complicated than that, as most things are -- someone once broke PR politics down for me but I forgot everything except, "outcome would be uncertain."
The outcome would be very uncertain. If you've ever talked to someone from PR, it seems to be a very split opinion. They already get citizenship, they don't have to pay federal taxes but they do get some federal tax money spent on them. They have local representation but they do not have true federal representation.
This really isn't an issue of Republican Vs Democrat as more as it is what the people of PR want.