First the South. The South, historically, has always voted 10-20% less then the rest of the Country. Given the GOP's strength in the South, and the GOP's growing weakness in the rest of the Nation, the GOP is looking at 10-20% vote disadvantage when it comes to a popular vote. The flip side of this is when the South votes 10-20% less then the rest of the Country, the electoral collage eliminates that 10-20% less people voting in the South from affecting who wins the National Election. Remember we only have ONE national election, everyone else is elected on a state wide basis (Governors and Senators) OR in districts drawn by States (Congressional seats as while as state legislature seats). These state based elections occurs at the same time as the Presidential Elections, but only the President is elected nation wide.
When the Electoral Collage was invented, different states had different eligibility when it came to who could vote (and no state permitted Slaves to vote, but slaves were counted in the census when it came to determining how many congressional seats each state had, this was viewed as unfair at the time period and the 3/5th clause was added to resolve this dispute). Now come the 1820s, almost every state had adopted universal white male suffrage (South Carolina was one exception to this rule), come 1866 the US and every state had made Suffrage universal for ALL Males. By 1921 this was extended to women. Dispute these increases in who could vote (and I am ignoring the South's efforts to prohibit Blacks from voting till the 1950s, on the grounds it was illegal but permitted) the South's problem of having lower voter turnout then the rest of the Country remained (And some of this lack of turnout was do to, and is do to, persistent racism in the South).
People in the South do NOT vote at the same level people vote in the rest of the Country. The South knows this, as to the Democratic and Republican Parities. The GOP prefer to accept this situation rather then try to get more voters out (Probably on the fear that more blacks will vote and the South would return to the Democratic Party). The South's excuse is that why should the south lose its existing vote count, based on its population NOT the number of people who vote, do to the fact the rest of the country vote at higher percentages then the South. The Electoral Collage provides votes to reflect the POPULATION of each state NOT how many voters vote in each state. The GOP knows this and will continue to refuse to have any GOP controlled State pass the compact. As long as the Compact needs 270 votes out of 525 electoral votes, the GOP will prevail. Remember the totals, The GOP controls (or more accurately can prevent this bill from getting to the Floor) of the following states with their electoral votes"
Texas (34)
Louisiana (9)
Mississippi (6)
Alabama (9)
Georgia (15)
South Carolina (8)
North Carolina (13)
Virginia (15)
West Virginia (5)
Kentucky (8)
Tennessee (11)
Arkansas (6)
Kansas (6)
Oklahoma (7)
Missouri (11)
Florida (27)
Then the Midwest (and I know Pennsylvania is considered an "Eastern State" but Pittsburgh and the Western Part of the State is Midwestern in outlook, not east coast). The Midwest is 180 Electoral Votes in Pennsylvania, Ohio, Indiana and Michigan. In each of these states the GOP still have strength to make sure this does not pass those states, so you have to add 69 more oppose to this compact( Total 249).
Finally the Great plains and the West (But NOT the West Coast) If you go into the Great Plains and West, you can add the following states to the states that will NOT pass this bill:
North Dakota (3)
South Dakota (3)
Montana (3)
Utah (5)
Wyoming (3)
Arizona (10)
Idaho (4)
Nebraska (5)
Alaska (3)
That is an additional 39 Electoral votes (42 if you include Nevada, which went for Obama in 2008, but will fear lost of power if popular vote becomes the law). ALL of these states (With the exception of Arizona) will not like the change for each will lose electoral vote power, for direct election eliminates the additional power these states have do to the fact the electoral vote goes by members in the House and Senate and each state gets at least two senator and thus two less votes given the state LOW population. This totals 288 electoral votes, which is more then the 270 needed under the terms of this act.
Thus unless Congress does something to force states to adopt this rule (Which I do NOT see happening, even if it was constitutional) this compact will never be adopted by enough state to pass. To many states have a vested interest in making sure it does NOT become passed by enough states, and in the middle you have the Midwest which probably would like this compact, but enough Republicans exist in those states to prevent such a compact from ever passing those states (More to keep the GOP viable on the National Level then anything in those states themselves).
For more details see:
http://en.wikipedia.org/wiki/United_States_Electoral_College