The Constitution specifically says that DC is NOT a state. An amendment is defiinitely needed to garner constitutionality. But I expect the Democrats to simply ignore the Constitution.
Your thinking of the 23rd ammendment. All that does is give DC electors even though it is not a state. It does not state that DC CANNOT apply for statehood. I’m just saying there is a way they can get senators and representatives while staying within the framework of the Constitution. The Dems are trying to ignore the Constitution and they really dont even have to do that. But they consider the Consitution irrelevant anyway so it doesn’t really surprise me.