If you are a US citizen you are the Union. Any state that may, in the future, join the Union would become US citizens. If you are referring to a workers union then it depends on the constitution of that union. There are several international ones.
Hawaii
As one of the 13 original colonies it gained admission to the Union by ratifying the US Constitution.
Daniel Webster
Once a state of the Union you are protected by the Constitution
It was the 8th State to ratify the Constitution and join the Union.
We the people in order to form a more perfect union
Because they saw benefit in joining the union.
That is the start of the preamble to the US Constitution.
The US Civil War ended with a Union victory, the dissolution of the Confederate States and the restoration of Union. By the end of 1865, the US Constitution was amended to abolish slavery.
The US constitution was ratified by the governments of the original 13 colonies. The remaining 37 states of the union implicity ratified the constitution just by applying to join the existing union./nine states
obviously it has worked well as it is the constitution with the longest tenure of its kind in the world. we are still a union, therefore, the constitution continues to be successful