When a state becomes unmanageable and violates the federal pact enough in frequency or degree such as (but not limited to): denial of republican form of government, nullification of federal laws, establishing sanctuary for illegal aliens, impedance of federal function or enforcements, reckless and incompetent fiscal practice into state bankruptcy, fostering insurrections, etc. then the House (by 2/3) and half of all state legislatures approving invokes the expulsion process of that state from the Union.
Following further through on this which has taken cue from the Constitution's amendments and impeachment clauses:
The Senate shall then have the sole power to try the expulsion with the Chief Justice presiding. The pro-expulsion forces within the House and state legislatures will present counsel their case for expulsion. The state in question will have final approval of their counsel to counter that case. No state shall be expelled without concurrence of 2/3 of the Senate.
Once a state is expelled, their congressional delegations will have no further representation on new legislation. A period of negotiations will ensue between the expelled state and the remaining United States over debts, federal installations, interstate infrastructures, boundaries, borders, expatriation, etc.