> The Constitution reserves the choice of the precise manner for creating electors to the will of the state legislatures. It does not define or delimit what process a state legislature may use to create its state college of electors.
Nevertheless, Article IV, Section 4:
> The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.
If a state is choosing electors based on voters not in the state, then the citizens of the state no longer have a republican form of government. The state cannot simply arbitrarily choose who to force their electors to vote for. The process must be a republican one somehow.