The Supreme Court's interpretation of the 10th Amendment is so strong that states cannot even voluntarily agree to be "commandeered" by the federal government to enforce federal programs. The principle of federalism is a structural rule that individual states cannot waive.
The Insurrection Act is a potent tool for executive power because it is a specific exception to the Posse Comitatus Act, which normally prohibits using the military for civilian law enforcement. Invoking it allows a president to deploy troops in American cities to perform police functions.
While Congress can use spending to incentivize states, this becomes unconstitutional coercion when the financial penalty for non-compliance is so severe it becomes a "gun to the head." This was the basis for striking down the Affordable Care Act's original Medicaid expansion provision.
The Constitution's framework for admitting new states (Article 4, Section 3) does not set any upper limit on the size of the union. This design implicitly allows for continuous growth and territorial expansion, a feature that contrasts with the nation's self-perception of not being an expansionist or colonial power.
When Texas installed river buoys, citing an "invasion" under Article 4's Protection Clause, the Biden administration strategically sued on mundane federal law (navigable waters). This avoided a potentially landmark and politically explosive Supreme Court ruling on what constitutes an "invasion" by a state.
A lesser-known principle, the "equal sovereignty" doctrine, posits that Congress cannot treat states unequally without compelling reason. While used to strike down parts of the Voting Rights Act, states could invoke it to argue they are being unfairly punished or targeted by a hostile federal administration.
The Full Faith and Credit Clause isn't just a command to states; it contains an "effects clause" granting Congress power to regulate how states recognize each other's acts. This was used in the Defense of Marriage Act (DOMA) to permit states to ignore same-sex marriages from other states.
Rather than reacting after the fact, a coalition of Democratic state Attorneys General has been actively planning and preparing legal challenges based on potential Trump administration actions detailed in documents like Project 2025. They aim to have complaints ready to file immediately, ensuring they are not "caught flat-footed."
Instead of the politically charged "sanctuary" label, California's Attorney General frames the policy as a strategic choice to focus limited state law enforcement resources on local crime, rather than federal civil immigration enforcement. This is positioned as a pro-community trust and public safety measure protected by the 10th Amendment.