From Article II of the United States Constitution:
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: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.(My emphasis.) At the very least the Democratic-led state legislature should make a strong attempt to keep this off the ballot.
Should that fail and should the initiative pass, I think that the wording in the Constitution would be enough to put the plan on hold while appeals are filed.
What are the chances of it passing? A new Field Poll shows 49% of state voters support the change, 42% oppose it.