It’s been 60 years since a new state entered the union, but now Democrats and liberals are accelerating efforts to gain statehood for Washington, D.C., and Puerto Rico. One of their motivations is the future of the U.S. Senate, which is currently biased toward the Republican Party. The logic goes that if Democrats can get unified control of the federal government after the 2020 election, they could push through statehood for both, adding four more seats to the Senate, and all four would likely be Democratic leaning.

That might seem far-fetched, but the U.S. has a rich history of partisan state-making.

Democrats have reason to worry about the Senate — their party is reliant on a coalition of voters predominantly situated in and around major population centers, making it harder to compete in rural states, which get the same number of senators regardless of population. Indeed, the more densely populated a state is, the more it tends to lean Democratic.

“It’s sort of a tilted playing field,” said Frances Lee, a political scientist at the University of Maryland who studies Congress’s upper chamber. “It’s easier for Republicans to elect a majority to the Senate than Democrats.”

Thirty-one states lean more Republican than the country as a whole, according to FiveThirtyEight’s partisan lean metric, so it’s easy to see why the party currently holds a 53-seat majority in the Senate.

Which brings us back to D.C. and Puerto Rico. Both would likely vote Democratic — D.C. is solidly Democratic in presidential elections and Puerto Rico would probably go blue, though that’s not a given. Theoretically, the two new states would give Democrats four more seats in the upper chamber. But first Democrats would need to win control of Congress (to get statehood legislation passed) and very likely the presidency (to sign the bill). The Constitution grants Congress the power to admit new states, but doesn’t say much else about the process for admission or what requirements new states have to meet. And passing such legislation won’t necessarily be easy, even if Democrats do gain full control of Congress.

Efforts to push D.C. and Puerto Rican statehood seem to be gaining momentum, but they’ve still been uneven. Over 200 House Democrats — most of the party’s 235-member majority — have signed on to a bill that would grant D.C. statehood. While it’s unlikely to pass in the GOP-controlled Senate, companion legislation has also been introduced there, with more than 30 Democrats backing it. The House and Senate bills seek to circumvent thorny constitutional questions about D.C. statehood — the Constitution gives Congress authority over the seat of government — by shrinking the federal district to encompass just the areas around the U.S. Capitol, the White House and the National Mall. The rest would become a new state called “Washington, Douglass Commonwealth,” in honor of Frederick Douglass. And Washingtonians definitely want statehood: 86 percent voted for it in a 2016 referendum. (As anyone who has seen a D.C. license plate knows, many residents aren’t happy about “taxation without representation.”)

Statehood for Puerto Rico actually has more support among the public than statehood for the District of Columbia, but congressional Democrats haven’t demonstrated the same level of commitment to making the island a state. Although the House has introduced statehood legislation for Puerto Rico, the bill has fewer than 20 cosponsors — less than one-tenth of the support the D.C. bill is getting. But in contrast to the D.C. legislation, the Puerto Rican statehood bill actually got some House Republicans to sign on. And while Puerto Rico doesn’t have any companion legislation in the Senate, Florida Republican Sens. Marco Rubio and Rick Scott have expressed support for the island’s statehood.

People advocating for Puerto Rican statehood may be able to drum up Republican support because some conservatives think that residents’ desire for autonomy within the U.S. federalist governing structure might actually make Puerto Rico less receptive to Democratic candidates. But Puerto Ricans have more mixed views about statehood than Washingtonians do. Although 97 percent of voters backed statehood in a 2017 referendum, only 23 percent of registered voters turned out, and a 2018 poll found that 48 percent of Puerto Ricans supported statehood compared to 26 percent who wanted to it remain a territory and 10 percent who wanted it to become an independent nation. Nonetheless, Gov. Ricardo Rossello is pushing hard for statehood.

But even if Democrats went all in on statehood for both D.C. and Puerto Rico, there are serious obstacles to achieving either. First, Democrats would have to win the presidency and Senate in 2020 while holding onto the House. And even if they manage that, the Senate requires a supermajority of 60 votes to get most things done. It’s nearly impossible to imagine Democrats gaining 13 Senate seats to get from the 47 they already have to the supermajority they’d need to pass bills without any Republican votes. So it may be difficult to corral enough bipartisan support to pass statehood legislation for Puerto Rico and, especially, D.C. Historically, Lee said, new states were often added in pairs (a slave and a free state, or a Democratic-leaning and a Republican-leaning state) to balance each other out, thus getting buy-in across party or sectional lines. As a result, Lee is skeptical that Democrats can get statehood for both D.C. and Puerto Rico because “anytime a proposal will benefit one party, you can expect it to be fought ferociously.”

Instead, Democrats might have to take the “nuclear option” by changing Senate rules to essentially end the filibuster — which some Democratic senators seem wary of doing. Still, it would not be the first time a party has tried to add states to boost its political standing in the Senate. Charles Stewart, a political scientist at MIT, has examined late-19th century Republican efforts to achieve statehood for territories that were expected to vote Republican. “Back then there was very active interest in manipulating the election system to partisan advantage,” said Stewart. “Although there is a lot of controversy about it these days, we ain’t seen nothing yet.”

During and after the Civil War, Republicans worked to bring in sparsely populated states such as Idaho, Nevada and Wyoming to help the party retain power. So from 1875 to 1897, the GOP controlled the Senate — which was elected by state legislatures back then — for nine out of 11 Congresses even though Democrats held the popularly-elected House eight times. As a result, Democrats achieved unified control of government (including the presidency) for just one of those 11 Congresses. This helped the GOP protect many policies it had put in place as the dominant party during the Civil War and Reconstruction. Plus, Republicans controlled both the presidency and Senate for more than half that time, enabling them to make favorable judicial appointments that helped preserve the party’s preferred laws.

With this history in mind, Democrats could take statehood politics to its logical extreme. Democratic Sen. Brian Schatz of Hawaii recently tweeted that American Samoa and Guam should get voting representation in Congress along with D.C. and Puerto Rico. Statehood for Guam might seem outlandish — as of the 2010 census, it had about 160,000 people, much less than even the least-populous current state, Wyoming, which was home to 560,000 people. But some states the GOP brought into the union during the late 19th century had far fewer people than the average House district. Nevada was particularly egregious — it became a state in 1864 but had an estimated population of only about 21,000 people, 17 percent of the average House district at the time, according to research by Stewart and his co-author Barry Weingast of Stanford University. As of 2010, the average House district had about 710,000 people, so Guam’s population would be equivalent to about 22 percent of the average district, comparably larger than Nevada’s in 1864. If Democrats could get full control, were willing to sacrifice the filibuster and really wanted to go all out, pursuing statehood for all U.S. territories might be an end that would justify the incredibly incendiary means.

However, if Democrats pursued such a course, Stewart says it might encourage Republicans to attempt countermeasures. He points out that Republicans in the 1880s pushed to split the Dakota Territory in two rather than bring it in as one state. “The next time the GOP controls everything, why not pass a law for Texas to split into five states?” Stewart asks. “Where does this end? That’s really the question, how partisan and how nasty are people willing to get.” If Democrats overcome the obstacles and get statehood for D.C. and Puerto Rico, we might find out.