In the March 2020 Atlantic, writer and attorney Simon Barnicle laid out one of the simplest ways to re-balance the Senate and Electoral College without a constitutional amendment:
Realizing that the deck is stacked against them, but recognizing that constitutional amendments are a pipe dream, some Democrats have called for structural reforms that could be accomplished with a simple majority in Congress: court packing, filibuster reform, and the legally dubious Senate Reform Act, to name a few. These proposals, while perhaps well intentioned, are inadequate. At best, they are temporary fixes—the minute Republicans regain control, they will retaliate in kind. And given the structural advantages enjoyed by Republicans, Democrats are unlikely to benefit in the long run.
A better solution to the problem of minority rule would address it directly. Democrats—if and when they regain control of Congress—should add new states whose congressional representatives would likely be Democrats. In areas that are not currently states, like Washington, D.C., or territories like Puerto Rico, this could be done with a simple congressional majority. But Democrats should also consider breaking up populous Democratic states and “un-gerrymandering” the Senate. Perhaps there could be a North and South California, or an East and West Massachusetts. A new state of Long Island, an area that is geographically larger than Rhode Island, would be more populous than most of the presently existing states.
In the short term, new Democratic states would remedy the advantages Republicans currently hold in the Senate—and, to a lesser extent, the Electoral College—which allow a party to control the federal government despite a lack of popular support. And unlike other progressive proposals, the risk of retaliation and escalation is low. Because adding states would also add Democratic senators, there would be no way for Republicans to immediately add states of their own without an overwhelming electoral victory.
[T]he federal government is increasingly acting on behalf of a smaller fraction of the population. And unless Democrats get serious about adding new states to counteract the Republican advantage, the disconnect between popular votes and control of the federal government is likely to grow.
My guess is that DC will become the 51st state before the end of the next Congress, possibly followed by Puerto Rico in 2023. But here we have to tread carefully: Puerto Rico would probably send one member of each party to the Senate, if not two Republicans. I'm willing to take that chance though.