The National Popular Vote bill would give a voice to the minority party voters for president in each state.  Now their votes are counted only for the presidential candidate they did not vote for. Now they don't matter to their candidate.   

In 2012, 56,256,178 (44%) of the 128,954,498 voters had their vote diverted by the winner-take-all rule to a candidate they opposed (namely, their state’s first-place candidate). 
				
And now votes, beyond the one needed to get the most votes in the state, for winning in a state, are wasted and don't matter to presidential candidates.  

In 2004, 
UT (5 electoral votes) alone generated a margin of 385,000 "wasted" votes for Bush. 
OK (7) alone generated a margin of 455,000 "wasted" votes for Bush - -- larger than the margin generated by the 9th and 10th largest states- NJ and NC (each with 15 electoral)
8 small western states, with less than a third of CA's pop, provided Bush with a bigger margin (1,283,076) than CA provided Kerry (1,235,659).