Doesn't seem likely to me. A more interesting proposal would be to have all but two of the electoral votes for each state determined by congressional districts, with the remaining two cast as the individual state legislatures opt to define. It would also be likely to reduce the need for recounts to merely a few districts instead of whole states. Such a proposal would probably benefit at least 38 states, and would thus stand a much greater chance of passage than California's proposal.
They are actually not concerned about the Constitution, because they intend to subvert the Constitution by the method of putting their proposal into affect.
Those states that adopt it will use it, and when, they hope, enough high population states have adopted it, it will have the desired affect on the election no matter how many small and medium population states have not adopted it.
It is a subversion of the Constitutional process because the method of adopting it does not require a mandate from 2/3s of the states and an act of Congress.
The "popular vote" idea would be a travesty for the Republic at some future point in time when just a few very high population states - could decide the election all on their own - no matter how few states they represented.
The "nation" is not simply the "population". It is the land, the states, the counties and the localities and the electoral college process requires that a winning candidate collect majorities in a majority of the local jurisdictions across the country, not just a handful of states.
Those that rightly see the error of the absolute tyranny of the majority must oppose this move.
This is a national movement. The Dems just need to pick off a few key states. Depending on the state, it may be possible for the voters or state legislatures to elect to join the compact.
EVERY VOTE EQUAL: A State-Based Plan For Electing The President By National Popular Vote
If the real intent was to make California relevant again, the simple solution would be for California to allocate electors the same way as Maine and Nebraska do-- two for the winner of the state, one for the winner of each congressional district.
While this method would hurt the DEMS in the short run by lessening the impact of multiple votes and other shenanigans in cesspools of corruption like San Francisco, Chicago or Philadelphia, it would enhance their chances in the long run by forcing them to run more moderate candidates with broader appeal.
NY and New England is all they need