The fight for D.C. statehood is hardly new.

It’s been decades since Congress first introduced legislation to make Washington, D.C., a state, and 27 years since such a bill got a full (losing) vote in the House of Representatives, but in late June, a historic step was taken: A majority in the House voted in favor of legislation that would make Washington, D.C., a state for the very first time.

Of course, this bill won’t be signed into law this year given the clear partisan calculus involved — making D.C. a state would almost certainly give Democrats two additional senators thanks to the District’s deep blue hue. But it’s important we understand why the Democrats are waging this fight now and why we might see more fights over admitting states in the years to come.

The answer boils down to unequal representation.

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On the one hand, the Senate has always been unequal, long giving less populous states an outsized voice relative to their population.1 But for more than a century, this hasn’t posed much of an issue: Until the 1960s, Republicans and Democrats competed for both densely and sparsely populated states at roughly the same rate

But over the last several decades, that’s changed. The parties have reorganized themselves along urban-rural lines, and there is now a clear and pronounced partisan small-state bias in the Senate thanks to mostly rural, less populated states voting increasingly Republican. In fact, it’s reached the point that Republicans can win a majority of Senate seats while only representing a minority of Americans.

One way to observe this growing partisan bias in the Senate is to compare the party makeup of senators elected to represent the 15 most populous states (which have collectively housed about two-thirds of population since the turn of the 20th century) to the partisan makeup of senators elected to represent the 25 least populous states (which have collectively housed roughly a sixth of the population consistently since the 1960s). As the chart below shows, the partisan makeup of the Senate was fairly even until the 1960s, when Republicans started to amass a partisan advantage in less populated states.2

The Senate Has Always Favored Smaller States. It Just Didn’t Help Republicans Until Now. 1

What happened? Much of this follows from the post-civil rights realignment of American partisan politics, in which the Democratic Party became more consistently liberal (and thus more appealing in big, largely urban states), and the Republican Party became more consistently conservative (and thus more appealing in small, largely rural states). But that gap has also widened in recent years, especially starting in 2015, when Republicans took back a Senate majority, flipping seats in small states like West Virginia, South Dakota, Arkansas, Alaska and Montana — all states that will be tough for Democrats to regain in 2020.

And what this has meant practically is that Republicans now hold a majority of Senate seats while only representing a minority of Americans, as you can see in the chart below.3

The Senate Has Always Favored Smaller States. It Just Didn’t Help Republicans Until Now. 2

This imbalance is significant because it poses a real obstacle to Democrats taking back a Senate majority in 2020. Take Democrats’ current odds of retaking the chamber. The Cook Political Report recently said Democrats are favored to win the Senate, but considering Democrats currently lead the generic ballot for Congress by over 8 percentage points and have a similar margin nationally in the presidential race, it’s remarkable that they still are only slight favorites to control the upper chamber.

Even if D.C. or Puerto Rico were states (as some on the left advocate), Republicans would still have the advantage. It’s true that the statehoods of D.C. and Puerto Rico would help Democrats close the small-state gap, but even if both were states and elected two Democratic senators, Republicans would still have had a two-seat majority in 2019, while only representing 48 percent of the population.

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The Senate has always held a contested place in America’s democratic system because of its non-proportional qualities. For the first half of the 19th century, the Senate was a bulwark for the South, with an equal balance of slave and free states despite the growing Northern population advantage. And in the second half of the 19th century, Republicans attempted to “stack” the Senate by admitting a large number of Republican states into the union, starting with Nevada in 1864 (population of just 6,857(!) in the 1860 census), Nebraska (1867), Colorado (1876), Montana, Washington, and North and South Dakota as separate states in an 1889 omnibus, and Idaho and Wyoming in 1890.

But despite rising prairie populism spreading through the Great Plains to the Mountain West in the 1890s, Republicans’ hopes for a stacked Senate didn’t work out quite as planned. And thanks to the way the American two-party system developed in the 20th century, with Democrats and Republicans both containing urban liberal and rural conservative wings, the small-state bias of the Senate never became a real partisan issue — until now. It will likely remain an issue, too, as long as one party is able to win a majority in the chamber while only representing a minority of the population.

Confidence Interval: Democrats could win 60 Senate seats | FiveThirtyEight