Is the National Popular Vote unconstitutional?

Without much fanfare, New York State this spring joined the movement to have presidential elections decided by a "National Popular Vote."  For latecomers, the NPV is a scheme to democratize presidential elections by taking advantage of the states' constitutional rite to decide how electoral votes are awarded. Currently, all but two states award electoral votes on a winner-take-all basis, based on the popular vote in each state. The other two states allow the popular vote in each congressional district determine which candidate gets that district's electoral vote. Under the NPV plan, states adopting NPV agree to award all their electoral votes to the candidate who wins the popular vote nationwide. NPV goes into effect only when states sufficient to form an electoral-vote majority adopt it. With New York's commitment NPV is a little over halfway home, but Professor Ian J. Drake of Montclair State University warns that the plan probably wouldn't survive a constitutional challenge. Drake's objection to NPV is twofold. He argues that the plan is unconstitutional because it "seeks to overturn the Electoral College without a constitutional amendment." This objection looks flimsy because the NPV is all about how states choose their electors. It abolishes neither electors nor the Electoral College. The constitution doesn't oblige states to award electors on a winner-take-all basis, or on a district-by-district basis. It doesn't oblige states to award electors on any basis tied to the popular vote in any given state. In the early days, some state legislatures appointed electors without reference to any popular vote for President. However, Drake has a potentially more substantial objection. His fallback position is that the NPV violates the Constitution's Compact Clause, i.e. "No state shall, without the Consent of Congress ... enter into any Agreement or Compact with another state." Current jurisprudence has clarified that Congressional approval is necessary if a compact among states "penalizes" states that aren't parties to it. In Drake's view the NPV falls into this category because "low-population states will be disadvantaged" by the loss of their current disproportionate yet constitutional electoral voice in presidential elections. But if this requires NPV to be submitted to Congress, then NPV is checkmated, because Congressional approval would amount to Congress making an unconstitutional law affecting the states' appointment of electors. Drake's defense against the NPV depends on it being recognized as a "compact," but he claims that NPV proponents "concede that [the several] state laws form an interstate compact." He gives no evidence of this, but what really matters is what the courts consider a compact, and it's unclear to me, as a constitutional layman, whether any state's provisional adoption of NPV amounts to joining a compact. I have no real dog in this fight, since the NPV would only further reduce the potential of independent parties to have a significant impact on presidential elections by forcing one to the House of Representatives. While the fight over NPV may be fought over typical partisan lines, only the two major parties really benefit, as things now stand, from the ultimate activation of the NPV plan. Nevertheless, at first glance the legal arguments against NPV advanced by Drake look relatively weak. Meanwhile, the political opposition to NPV seems to have not yet begun to fight. When it begins to look as if one or two states could tip the balance nationally in favor of NPV, then the real debate, and the expensive ad campaigns, will start.