Final answer:
A congressional act requiring territory residents to draft a state constitution before being admitted to the Union is outlined by the Northwest Ordinance of 1787 and was a key step in the statehood process historically.
Step-by-step explanation:
A congressional act directing the people of a territory to write a state constitution as a step toward admission to the union is part of the process of statehood as outlined historically by documents like the Northwest Ordinance of 1787. This document set out criteria for the transition from a territory to a state within the United States, requiring a territory's population to reach a certain threshold before they could draft a constitution and seek admission as a state.
Once a territory's population exceeded 60,000, a territory was eligible to draft a constitution which ensured certain rights including forbidding slavery and protecting religious freedom, after which they could apply for statehood and, if approved by Congress, be admitted into the Union.
Additionally, during the Reconstruction period following the Civil War, similar measures were taken to reintegrate Confederate states back into the Union. The process required states to draft new constitutions that guaranteed certain rights, such as black suffrage, in order to be readmitted.