Under this compact arrangement, the voter does not know how that vote will be counted. The Electoral College vote in Massachusetts will not be based on the popular vote in Massachusetts. It will be based on the number of dead people who vote in Chicago. The current system has a clear result for a counted vote, so the voter knows.
Imagine a future election where the Democrat wins Massachusetts with 80%, but the Republican wins a squeaker with 50.1% of the national vote. You mean to tell me that the Supreme Court of Massachusetts is not going to figure out a way to let the Legislature cast those votes for the Donk favored by 80% of their population?
Eventually the matter would be settled in court, but there would be a heck of a lot of pushing and shoving before that happened.
Here is another nightmare scenario for you... Currently the Electors do not have to vote for the candidate they are "supposed" to vote for. In 2000, at least one Elector was threatening to change his vote, but it would not have made a difference, so nobody paid much attention. But how are you going to find an elector in Massachusetts who is going to go against "The Will of the People"?
The voter expects the vote to be added to the national popular vote, and that the state's Electoral College votes will be given to the national popular vote winner. You supposed that California (or Massachusetts) would balk at giving their Electoral College votes to a Republican if that candidate won the national vote (but not the California vote). I'm saying that if California, under the conditions above, reneg on the compact and give their votes to the Democrat anyway, then that is a violation of how the voter expected that vote to count when entering the voting booth, which would be illegal and could be contested.
The state legislature would have to pass a bill allowing for this compact before the presidential election first, so that the people know that this is the counting method for their presidential vote.
As for the issue of faithless electors, some states have laws binding the vote of electors, other states do not bind electors but have slates of supposedly trusted electors. I'd hate to be a trusted elector who betrays the state and turns the election the other way. Is it worth have one party love you will the rest of the country despises you?
-PJ -PJ