[Sherman Logan]

Puerto Rico can enter if Texas can split into 4 states.

TX can split into any number of states, as can any other state.

All it needs is for the state legislature and Congress to agree.

If any party had a sufficiently large margin in Congress and a number of states, it could split the each state it controlled into a large number of new states. This could in theory permanently pack both the House and the Senate. Perfectly constitutionally.

[Verginius Rufus]

In theory other states can split (it has happened before with Massachusetts in 1820 and Virginia under questionable circumstances in 1863), but in the case of Texas it was agreed when they joined the US that they could divide into as many as five states. Of course in 1845 the territory of Texas was quite a bit larger than it is now.

There were a number of proposals over the years to divide Texas but none of them got anywhere. One practical problem--how could all five states have the Alamo?