That's precisely what they want; however there are two problems with that. First the constitution only allows the state legislature to determine the method by which electors are chosen (not referenda). Second, even if referend were allowed, this one is on the date that the electors are chosen. According to federal election law, the method by which a state selects electors must be written in staute at least six days prior to the date the electors are chosen. This referendum could be designed to create even more controversies by creating the potential of Supreme Court decsions affecting how the state allocates its electors.
One article I read indicated that the proponents might decide whether to apply it to the 2004 election depending on whether it helps Kerry or not. Abviously if Kerry carried Colorado, DemocRATS would be shooting themselves in the foot if they applied it this year, and might cause Kerry to lose the Electoral College. If Kerry doesn't carry the state, the DemocRATS would be likely to want the law to apply this year. Of course that would be strange, because if Bush wins, it is unlikely for the referendum to pass.