What does the future hold for Donald Trump? - threadbans in OP

It goes back to the nature of the Federal District as not being one of the United States (pretty much by definition). Back in The Day, citizens who lived in DC did not have any representation at any level, to include local government which was run by appointees by the Federal Government, That didn’t sit well with the residents on the SouthWest side of the Potomac in particular, who felt their needs were not being addressed. Thus the retrocession of part of DC to Virginia in 1847, which also sets the precedent for how folks on the NorthEast side of the the river can gain congressional representation if they really want it. Of interest, however, the citizens on that side of the water decided against retrocession when the Virginian side voted in favor.

Interestingly, there is much less opposition from Republicans on the idea of Puerto Rico becoming a state as it’s an entirely different situation. The issue there is simply that Puerto Rico hasn’t asked to become one. It’s difficult to say where Puerto Rico would fall in Congress as their political parties mainly divide on the matter of independence, statehood, or status quo. The current Congressional representative caucuses with the Republicans. The previous went Democrat. The one before that, Republican. All three represent the same party, the New Progresive Party (the pro-statehood party). The one before those three caucused Democrat, but was a member of the “status quo” party, Popular Democratic.

Yes and no. If DC were overwhelmingly Republican, I have no doubt that in practice it would today be more Republicans talking about finding a way to turn most of DC into a State and Democrats arguing against. In this case, as a matter of principle, precedent is very definitely on the side of the Republicans: An active decision was made at the time the District was created that the people who lived there would in effect be kicked out of the States and thus lose their Congressional representation, with the "fix" being retrocession, as implemented for Alexandria County. The current arguments for "turn DC into a State" have really been vocally proposed only in recent years, after the 1980s.

PR is an entirely different principle and the positions of both parties have generally been the same over the decades: That if the PRians ask, they'll be admitted, this has been a constant position, which is presumably why the NPP has been willing to caucus both with Republicans and Democrats. The fact that the prospective state seems unlikely to tilt overwhelmingly one way or the other is probably why neither party has changed its historical position for pragmatic (i.e senate weight) reasons, but even if they wanted to, I'm not sure there are any arguments against.