The States are supposed to be guaranteed a republican form of government. Looks like a deal-breaker to me.
It's a crap proposal, a crap idea, and not enforceable. The idea of this is akin to throwing fuel on fire. It could lead to serious disruptions.
Very good question. I should think the answer is yes. If the rats want another civil war, this is the way to do it. It's a deal-breaker.
The Electoral College is selected by means determined by the State legislature. Until as late as 1820's some States selected their electors without regard to the "popular" vote. The intent of the Founders was to give the States a voice in selecting the Chief Executive, who then was answerable to the States, not to the "people," who still don't really vote for President.
This was part of a system once known as "federalism."
"If the deal is broken, should the states be free to leave the union?"
Thats the thing, the deal is NOT broken...
Except in principle.
And this is where the civil war begins. Liberals like to exploit the benefit of the doubt. If the movement can claim that they indeed have not broken the electoral college compromise (which very few people know about, nor will recognize if it militates against their political interests) then they will exploit the benefit of that ignorance and doubt.
But first it will end up in the supreme court as an 'in principle' constitutional violation.
Oh, no, a state can NEVER leave the Union. No matter if the Union lied or the SCOTUS simply changes the meaning of the Constitution, it's a contract that binds on you and your children, unlike any other contract you can sign, and doesn't bind the Federal Government to squat, unlike any other contract IT can sign. The Founders signed the lives of their progeny away, you see.
A very interesting question. You are right it would be a broken promise, not that that has ever stopped politicians from doing what they want.