If the Democrats win the presidency and both houses of Congress that year.
D.C. will be first in line, then Puerto Rico.
Is there any basis for DC gaining statehood? Where is it coming from besides random grumbling.
Wildly guessing: the federal government's response to Hurricane Maria hitting Puerto Rico was wildly insufficient, so if the opposition party takes control (which seems more likely than not), especially given that the opposition party prefers to err on the side of too much government aid/intervention than too little, statehood for Puerto Rico is a reasonably obvious priority. (And a goos strategy for shifting the balance of power in the Electoral College and the Senate, incidentally.)
Except Puerto Rico has to advance a state constitution that Congress has to accept, so the Puerto Rico government itself needs to accept statehood. Except there's not a clear majority in favor of statehood in Puerto Rico; the last two referendums were effectively sabotaged.