It's to give smaller states a say.
. . .
Now political clout comes from being among the handful of battleground states. 80% of states and voters are ignored by presidential campaign polling, organizing, ad spending, and visits. Their states’ votes were conceded months before by the minority parties in the states, taken for granted by the dominant party in the states, and ignored by all parties in presidential campaigns.
State winner-take-all laws negate any simplistic mathematical equations about the relative power of states based on their number of residents per electoral vote. Small state math means absolutely nothing to presidential campaign polling, organizing, ad spending, and visits, or to presidents once in office.
In the 25 smallest states in 2008, the Democratic and Republican popular vote was almost tied (9.9 million versus 9.8 million), as was the electoral vote (57 versus 58).
In 2012, 24 of the nation's 27 smallest states received no attention at all from presidential campaigns after the conventions. They were ignored despite their supposed numerical advantage in the Electoral College. In fact, the 8.6 million eligible voters in Ohio received more campaign ads and campaign visits from the major party campaigns than the 42 million eligible voters in those 27 smallest states combined.
The 12 smallest states are totally ignored in presidential elections. These states are not ignored because they are small, but because they are not closely divided “battleground” states.
Now with state-by-state winner-take-all laws (not mentioned in the U.S. Constitution, but later enacted by 48 states), presidential elections ignore 12 of the 13 lowest population states (3-4 electoral votes), that are non-competitive in presidential elections. 6 regularly vote Republican (AK, ID, MT, WY, ND, and SD), and 6 regularly vote Democratic (RI, DE, HI, VT, ME, and DC) in presidential elections.
Similarly, the 25 smallest states have been almost equally noncompetitive. They voted Republican or Democratic 12-13 in 2008 and 2012.
Voters in states that are reliably red or blue don't matter. Candidates ignore those states and the issues they care about most.
Kerry won more electoral votes than Bush (21 versus 19) in the 12 least-populous non-battleground states, despite the fact that Bush won 650,421 popular votes compared to Kerry’s 444,115 votes. The reason is that the red states are redder than the blue states are blue. If the boundaries of the 13 least-populous states had been drawn recently, there would be accusations that they were a Democratic gerrymander.
Support for a national popular vote is strong in every smallest state surveyed in recent polls among Republicans, Democrats, and Independent voters, as well as every demographic group
Among the 13 lowest population states, the National Popular Vote bill has passed in 9 state legislative chambers, and been enacted by 4 jurisdictions.
Now, the 11 most populous states (with over 270 electoral votes), by themselves, containing 56% of the population of the United States, could determine the Presidency.
