so far this is local to the individual state.
can the state require them to IGNORE their local voters to comply with the national popular vote? I would think that violates the constitution.
(ie. if calafornia had 300 million voters then the other 49 states and territories would have to be mandated to vote with california)
Only if those other 49 states chose to.
Quite the contrary, it would be completely in keeping with the Constitution. Article II, Section 1: "Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress..."
If a state wants to allocate it's electoral votes to the popular vote winner it can. If a state wants to end popular election and let the legislature alone decide who gets the electoral vote it can. The people of South Carolina voted for a presidential candidate for the first time in 1868.