I see there is some interest in the issue of statehood as it relates to contemporary hardball politics. I wrote a full dissertation on the topic! Here's a short article I wrote that distills my research. I'll give the elevator pitch in this tweetstorm.
mattglassman.com/wp-content/upl…
mattglassman.com/wp-content/upl…
Statehood is truly hardball politics. By simply passing a federal law---bare majorities in the House and Senate, plus the signature of POTUS, you can irreversibly create a state and alter the electoral composition of both Congress and the electoral college.
Right now, assuming they were willing to use the nuclear option to abolish the filibuster for state admissions, the GOP could make Puerto Rico or DC a state or (with the consent of the state leg) divide Texas (or Wyoming) into any number of states. WIth just a law. Irreversibly.
People don't appreciate how much this impacted 19th century politics. It was an issue at the convention as the delegates comtemplated representational security in a growing nation. It was a part of every major slavery compromise. And it was as political cudgel used constantly.
The map of the West---which structures representation and thus public choice in the federal government---was the byproduct of a profoundly intense local and national political struggle that lasted a century. The number, size, and shape of states is a footprint of that politics.
That said, people badly misunderstand statehood politics. They think of it as a congressional fight over admission of states, made controversial by its role in the slave crisis and structured by the Balance Rule. This is a bad way to think about statehood. Let's get started.
What most people know of statehood politics is the Balance Rule, the informal mechanism of pairing state admissions by slave and non-slave states, to preserve a balance in the Senate. It's a useful---but very incomplete---analytical explanation of the politics.
For one, it complete fails to explain state admissions in the 1850s. Take Oregon in 1858. Its admission runs counter to every feature of the balance rule. The Southern Democrats defeat the Republicans in Congress to bring a free state into the Union! 
My argument is that the Balance Rule thinks far too narrowly about the politics of statehood, and thus misses its crucual features. First, statehood was inherently controversial without reference to slavery. Slavery just happened to be the major political cleavage of the day.
But statehood politics is a dogfight both before and after slavery is the dominant political cleavage. It's an issue at the constitutional convention, and it's an issue long after the civil war settles the slave crisis. Differnet interests at those times, but similar politics.
Second, statehood is much more than a battle over admissions. That's the *final* stage. Prior to that are political battles over creating and shaping new terrirtories. Often, these are much more important than the final admission politics.
By the 1850's, the recognition that territorial politics creates path-depenent states pulls the Balance Rule compromises away from admission an toward territorial creation. Othe prizes may be used to balance interests, such as public policy. Kansas-Nebraska is a good example.
Third, statehood politics is not just a politics in Congress. Local western politics play a key role in the creation/division of territories, and from early on Congress has a norm that territories would not be admitted against their will by Congress fiat, giving them a veto.
This gums up the Balance Rule, of course, when Kansas rejects the English Bill Constitution in 1858. But more importantly, western interests strongly shape the final size, number, and shape of new states. Here's an old thread on the division of Dakota.
Fourth, the constittional statehood process is ridiculous. Creating a state is perhaps the only utterly irreversible powers in the Constitution, it's easy to do, and there are no qualifications on it. This is why it got consumed by hardball politics in the 19th century.
As it turns out, the Founders *wanted* a structure to the process---they passed the Northwest Ordinance!---but they made a serious constitutonal error. Here's the story:
And here is a comparison of six proposed plans for how to add new states.
As a result, throughout the 19th century, there was serious contestation over the statehood *process* itself, with constant proposals to reform it. Even the famous Crittenden and Douglas plans for constitutonal amendments to avert secession and war spend significant time on it.
Anyway, read the article. It's a fascinating topic, and the stories are great. Also, here's a blog post I did about Arizona and New Mexico territories and how their creation relates to the civil war:
mattglassman.com/?p=2814
mattglassman.com/?p=2814
And here's a more general blog post about the war and the final development of the western territorial boundary lines:
mattglassman.com/?p=1689
mattglassman.com/?p=1689
