Can you point me to a website or provide a link where the “this is constitutional” rationale is spelled out?
No, I can tell you. The constitutional system of electoral votes is not changed by this. Even under the compact, each state is picking it's prescribed set of electors and they are voting for the President. All that's changed is the method by which the electors are chosen. And there, the constitution gives the state legislatures carte blanche. Each state may pick its electors by whatever method each decides. They don't have to even hold an election and could instead simply pass a resolution giving the state's electors to a candidate.
Since this scheme only changes how states pick their electors, not what the electors do or how the system works, there's nothing unconstitutional about it.
“this is constitutional” rationale is spelled out
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I guess the opposite is “show where it is banned”.
https://en.wikipedia.org/wiki/Twelfth_Amendment_to_the_United_States_Constitution
The Constitution only specifies how many electors a state is allocated, it does not say how the states are supposed to select them. States can select their electors any way they choose.