Even if Democrats regain unified control of the White House and Congress in 2020, the fate of their ambitious legislative agenda will still likely hinge on a fundamental question: Do they try to end the Senate filibuster?

If the party chooses to keep the filibuster, it faces a daunting prospect: Democrats elected primarily by voters in states at the forefront of the country’s demographic, cultural, and economic changes will likely have their agenda blocked by Republican senators largely representing the smaller, rural states least touched by all of those changes. In fact, since the Senate gives each state two seats, the filibuster allows Republican senators from states representing only about one-fifth of the country’s population to be in position to stymie Democratic legislation.

Such considerations led Reid, in a striking op-ed last week, to urge Democrats to abolish the filibuster when they regain power—and led Senate Majority Leader Mitch McConnell to respond with an op-ed of his own defending the rule.

Democrats and Republicans alike have become more frustrated with the filibuster as the Senate has grown more polarized since the 1980s. While defenders of the procedure have portrayed it as a mechanism that forces the majority party to negotiate with the minority party, in contemporary Washington, members of both parties have found it difficult to attract much support from the other for its initiatives under any circumstance. As a result, the filibuster has become simply a means for the minority to block the majority rather than a spur to compromise.  […]