How do new states become part of the U.S.?

Demonstrators demand statehood outside a San Juan hotel where then-VP Dick Cheney was speaking in 2004. In 2012, a referendum on the island showed statehood received the most votes for the first time ever. See more protesting pictures.

On Nov. 6, 2012, Puerto Rico held elections, but not for president of the United States. Puerto Rico is a self-governing commonwealth that is "associated" with the U.S. Thanks to a 1917 law, Puerto Ricans are U.S. citizens, but since Puerto Rico isn't a state, the island's inhabitants cannot vote in presidential elections. In an odd twist, they can vote in presidential primaries, because primaries are organized by the Democratic and Republican national committees, not the federal government [source: Helgesen].

The 2012 Puerto Rican election was for governor, the highest-ranking official on the island, but it was a ballot referendum that grabbed the headlines. For the first time since Puerto Rico became a commonwealth in 1952, a majority of Puerto Ricans voted against the current relationship with the U.S. When asked what political status they preferred, "statehood" received the most votes [source: Fabian]. The popular vote for statehood represents a significant shift in Puerto Rico's political winds, but it's not enough to make the commonwealth a state.

According to Article IV, Section 3 of the U.S. Constitution, only Congress has the ability to authorize the creation of a new state. Specifically, Section 3 prohibits the creation of new states from the territory of others without their consent, or the combining of two or more states without Congressional approval [source: Monk].

In practice, a majority of the 50 states began as U.S. territories. Either the territories petitioned Congress for permission to draft a state constitution and elect representatives, or they followed the more aggressive "Tennessee Plan." Tennessee, the very first U.S. territory, was rejected in its first bid for statehood, but decided to draft a state constitution anyway, and send two senators and a representative to Congress [source: Lehleitner]. Although Congress refused to seat the men, they successfully lobbied for statehood. Several other states followed the Tennessee model, including Alaska.

Official admission to the Union requires Congress to draft -- and the president to sign -- a bill called an "enabling act." For Puerto Rico to become a state, it would need to convince Congress and the president that statehood is not only in the best interest of the Puerto Rican people, but in the best interest of the United States as a whole. The U.S. Senate and the House of Representatives would need to approve the statehood admission by a two-thirds majority vote [source: The Week].

For some perspective, let's look at where the other 50 states came from, and how the United States grew from 13 coastal colonies to the world's third-largest country.