One thing to remember is that even if the compact passed, there might be a serious constitutional challenge in the courts:
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"The constitutional foundation of NPVIC is Article II, Section 1 of the Constitution, which provides that states shall appoint electors "in such Manner as the Legislature thereof may direct." Advocates of the compact point out that the plain language of this text appears to provide legislators with plenary authority over the method of selecting electors, an interpretation endorsed by the US Supreme Court more than a century ago in McPherson v. Blacker in 1892 and again in 1969, Williams v. Virginia Board of Elections. Like all provisions of the Constitution, however, this section must be read in context and in conjunction with other provisions of the Constitution.
The principal constitutional impediment to NPVIC probably is the so-called "Compact Clause" in Article I, Section 10 of the Constitution, which provides that "No State shall, without the Consent of Congress ... enter into any Agreement or Compact with another State." Although the US Supreme Court has concluded that the Compact Clause does not require Congress to consent to compacts that affect only the internal affairs of the compacting states, it has indicated in US Steel Corporation v. Multistate Tax Commission that the Compact Clause requires Congress to consent to an agreement that "would enhance the political power of the member States in a way that encroaches upon the supremacy of the United States," or "impairs the sovereign rights of non-member states."
Advocates of the compact contend that it would not need congressional authorization because it does not encroach upon national supremacy. But while the compact might not literally interfere with the supremacy of the federal government, the compact would powerfully affect the federal government since it could change the outcome of a presidential election. It also would involve more than merely the internal affairs of the states since it would interfere with the federalist structure of the US Constitution's procedure for electing a president. The equal representation of every state in the Senate is an integral part of the fabric of the Electoral College insofar as it gives every state an electoral vote that is the total of the number of both its senators and its representatives. Although the compact would not violate the letter of the Constitution since it would retain the Electoral College and would not alter the method by which electoral votes are assigned or change the number of electoral votes that any state has, it would jettison the federalist structure of the Electoral College to the extent that the popular vote rather than the votes of individual states would determine the outcome. The compact's reduction of the Electoral College to an empty shell would therefore thwart the intention of the Framers of the original Constitution and the framers of the Twelfth Amendment, which reformed the Electoral College in 1804, since the Constitution clearly contemplates that electoral votes will be cast by the states as states rather than by the states as collective or compacting entities. Although this procedure permits (but does not require) individual states to select their electors on the basis of popular vote, it does not authorize the election of the president by any kind of national popular vote.
The Supreme Court has made clear that states may not enact legislation that interferes with the federalist structure of the Constitution, even when the Constitution does not expressly prohibit such legislation and even when a literal interpretation of the Constitution could support such legislation. In US Term Limits, Inc. v. Thornton, the Court held that states could not limit the terms that US representatives could serve even though the Constitution does not prohibit such limitations, because representatives are officers of the federal government. The Court explained that term limits would permit states to circumvent the constitutional provisions that allow Congress to determine the qualifications of its members and that the Framers did not spend "significant time and energy in debating and crafting Clauses that could be easily evaded" and "manipulated out of existence." Similarly, the Court might determine that the compact evades the Electoral College by trying to manipulate it out of existence.
The NPVIC is also vulnerable under the Compact Clause insofar as it would interfere with the interests of states that did not subscribe to the compact. Advocates of NPVIC deny that the compact would interfere with the rights of these states because an election's outcome would be based upon the total of votes in all states, including non-participants. The compact, however, would deprive such states of their ability to help to determine (or even to determine, if the electoral vote of one state would tip the election) the outcome of a presidential election on the basis of electoral votes cast individually by the states rather than collectively based upon a national popular vote. Since this would constitute a virtual abolition of the Electoral College, non-compacting states would be denied their right to participate in a constitutional amendment process to determine whether the nation should make such an important change in its method of selecting the president.
Since Congress could consent to the compact by majority vote rather than by the two-thirds majorities required by the constitutional amendment process, it is more likely that Congress would consent to NPVIC than that it would consent to a constitutional amendment to abolish the Electoral College. Congressional consent for NPVIC is far from assured, however, particularly because Congress might not appreciate its exclusion from a process that would significantly alter the method of selecting the president. If Congress withheld its consent, the constitutionality of the compact would need to be tested in court."
http://www.jurist.org/forum/2012/02/william-ross-vote-compact.php
***
I am not necessarily endorsing the constitutional arguments against the NPVIC, but they are not frivolous, and I am by no means certain that the Supreme Court would reject them.
***
"The constitutional foundation of NPVIC is Article II, Section 1 of the Constitution, which provides that states shall appoint electors "in such Manner as the Legislature thereof may direct." Advocates of the compact point out that the plain language of this text appears to provide legislators with plenary authority over the method of selecting electors, an interpretation endorsed by the US Supreme Court more than a century ago in McPherson v. Blacker in 1892 and again in 1969, Williams v. Virginia Board of Elections. Like all provisions of the Constitution, however, this section must be read in context and in conjunction with other provisions of the Constitution.
The principal constitutional impediment to NPVIC probably is the so-called "Compact Clause" in Article I, Section 10 of the Constitution, which provides that "No State shall, without the Consent of Congress ... enter into any Agreement or Compact with another State." Although the US Supreme Court has concluded that the Compact Clause does not require Congress to consent to compacts that affect only the internal affairs of the compacting states, it has indicated in US Steel Corporation v. Multistate Tax Commission that the Compact Clause requires Congress to consent to an agreement that "would enhance the political power of the member States in a way that encroaches upon the supremacy of the United States," or "impairs the sovereign rights of non-member states."
Advocates of the compact contend that it would not need congressional authorization because it does not encroach upon national supremacy. But while the compact might not literally interfere with the supremacy of the federal government, the compact would powerfully affect the federal government since it could change the outcome of a presidential election. It also would involve more than merely the internal affairs of the states since it would interfere with the federalist structure of the US Constitution's procedure for electing a president. The equal representation of every state in the Senate is an integral part of the fabric of the Electoral College insofar as it gives every state an electoral vote that is the total of the number of both its senators and its representatives. Although the compact would not violate the letter of the Constitution since it would retain the Electoral College and would not alter the method by which electoral votes are assigned or change the number of electoral votes that any state has, it would jettison the federalist structure of the Electoral College to the extent that the popular vote rather than the votes of individual states would determine the outcome. The compact's reduction of the Electoral College to an empty shell would therefore thwart the intention of the Framers of the original Constitution and the framers of the Twelfth Amendment, which reformed the Electoral College in 1804, since the Constitution clearly contemplates that electoral votes will be cast by the states as states rather than by the states as collective or compacting entities. Although this procedure permits (but does not require) individual states to select their electors on the basis of popular vote, it does not authorize the election of the president by any kind of national popular vote.
The Supreme Court has made clear that states may not enact legislation that interferes with the federalist structure of the Constitution, even when the Constitution does not expressly prohibit such legislation and even when a literal interpretation of the Constitution could support such legislation. In US Term Limits, Inc. v. Thornton, the Court held that states could not limit the terms that US representatives could serve even though the Constitution does not prohibit such limitations, because representatives are officers of the federal government. The Court explained that term limits would permit states to circumvent the constitutional provisions that allow Congress to determine the qualifications of its members and that the Framers did not spend "significant time and energy in debating and crafting Clauses that could be easily evaded" and "manipulated out of existence." Similarly, the Court might determine that the compact evades the Electoral College by trying to manipulate it out of existence.
The NPVIC is also vulnerable under the Compact Clause insofar as it would interfere with the interests of states that did not subscribe to the compact. Advocates of NPVIC deny that the compact would interfere with the rights of these states because an election's outcome would be based upon the total of votes in all states, including non-participants. The compact, however, would deprive such states of their ability to help to determine (or even to determine, if the electoral vote of one state would tip the election) the outcome of a presidential election on the basis of electoral votes cast individually by the states rather than collectively based upon a national popular vote. Since this would constitute a virtual abolition of the Electoral College, non-compacting states would be denied their right to participate in a constitutional amendment process to determine whether the nation should make such an important change in its method of selecting the president.
Since Congress could consent to the compact by majority vote rather than by the two-thirds majorities required by the constitutional amendment process, it is more likely that Congress would consent to NPVIC than that it would consent to a constitutional amendment to abolish the Electoral College. Congressional consent for NPVIC is far from assured, however, particularly because Congress might not appreciate its exclusion from a process that would significantly alter the method of selecting the president. If Congress withheld its consent, the constitutionality of the compact would need to be tested in court."
http://www.jurist.org/forum/2012/02/william-ross-vote-compact.php
***
I am not necessarily endorsing the constitutional arguments against the NPVIC, but they are not frivolous, and I am by no means certain that the Supreme Court would reject them.