Who has the authority to admit new states into the Union?

The authority to admit new states into the Union lies with Congress as outlined in Article IV, Section 3 of the United States Constitution. This section explicitly grants Congress the power to admit new states, which was a key aspect of the nation's expansion and development. The process typically involves the new state drafting a state constitution and obtaining approval from Congress, highlighting the legislative body’s role in managing the admission process.

The other entities mentioned do not have the constitutional authority to admit states. The President may influence the process or support statehood efforts but does not have the power to unilaterally admit a new state. The Supreme Court's role is to interpret laws and ensure they align with constitutional principles, without the direct authority to handle state admissions. State legislatures, while they might play a part in expressing their citizens’ will regarding statehood, do not have the constitutional mandate to grant statehood on their own. Thus, Congress stands as the sole authority empowered to admit new states into the Union.