[Non-Sequitur]

A vote of their representatives in Congress would be a federal act, and thus must fall within the enumerated powers of the federal legislature.

Why? Is it not the states in Congress assembled? Do they not represent the will of the people of those states? Such a vote would still express the will of the states and, since not forbidden, should be able to expel Massachusetts. Shouldn't it?

The Constitutional acid test that determines when action is constitutionally derived currently lies with the Supreme Court via their usurpation of this power in Marbury v. Madison.

Where did the Marbury v Madison decision state that?

[Gianni]

[NS, #736] Why could not the states, through a majority vote of their members in Congress, expel Massachusetts?

[NS, #738] Is it not the states in Congress assembled?

It is. The states in Congress assembled are a federal body.