An article (pdf) by Jonathan Turley in the George Washington Law Review argues convincingly that the DC voting rights bill is unconstitutional (that’s easy), and further that DC cannot constitutionally be made into a state either.
The problem can be resolved legally only by moving the residents of the District of Columbia into a state (such as by retrocession), or by a constitutional amendment. Retrocession seems preferable to me, as it would fix not only the problem of representation, but also the more general problem of which representation is an symptom: the bizarre special status of DC.
An obstacle to retrocession is the fact that years of mismanagement make Washington, D.C. an unattractive addition to any state. That obstacle could possibly be resolved through a federal pledge to cover Maryland’s DC-related costs for several years to come.
Either way, retrocession or amendment, Democrats will have to lower their sights. Retrocession would add one Democratic representative (in Maryland), but it would end the DC representation debate. It would not set a precedent that would lead ultimately to two additional Democratic senators. The same is true of any amendment that would be likely to pass. For that reason, DC representation will not happen until all the other schemes (unconstitutional but more attractive to Democrats) have been conclusively eliminated by the Supreme Court.