Careful what you wish for....
How is this blatantly unconstitutional? It is a strange idea, but there is a difference between being a bad or silly idea and being unconstitutional. The Federal Constitution left the method of the selection of the Electors of the Electoral College completely up to the discretion of each individual state. If California wants to change its election law and throw its votes to some random person, its their right.
Its actually rather surprising that all of the Republicans aren't backing the Democrats support of this bill. Afterall, California almost certainly will lean to the Democrat candidate and thus giving California's electoral college's votes to a third party will make it more likely for a Republican to win. There is however the possibility that no candidate will receive a majority in the electoral college and thus the House will choose the President. In that case whoever controls the most state delegations in the House will win. (If the House chooses the President, Each state delegation gets one vote as called for by the XII Amendment).