Opponents of the electoral college reached an important milestone last month.

Gov. Lincoln Chafee (D-R.I.) signed a bill into law that committed Rhode Island to the National Popular Vote interstate compact. That's a deal wherein states commit to send their electoral votes to the presidential candidate who wins the popular vote — but only once states representing over half of all electoral votes adopt similar laws. Once that threshold is reached, the electoral college is effectively abolished, without a constitutional amendment.

Rhode Island's addition means that the National Popular Vote plan — first conceived of by Northwestern's Robert Bennett and developed by Akhil and Vikram Amar at Yale and UC Davis, respectively — is halfway to its goal. As plan supporter Rick Hertzberg at the New Yorker explains, nine states plus the District of Columbia have now signed on, representing 136 electoral votes. That's 50.4 percent of the votes needed for the plan to come into force and for the electoral college to be abolished.

But there's a catch. If you look at the nine states that have passed the plan (not to mention D.C.), all of them did so when both houses of their state legislatures were run by Democrats. All but Hawaii had Democratic governors, and the Hawaii legislature overrode the Republican governor's veto. There's nothing inherent in the proposal that gives Democrats an advantage, and plenty of Republicans — like former Sens. Fred Thompson (R-Tenn.) and Jake Garn (R-Utah) and former Gov. Jim Edgar (R-Ill.) — support it. But since the electoral college cost Al Gore the presidency in 2000, abolishing it has come to be perceived as a Democratic priority.

As long as that's still the case, the National Popular Vote compact just isn't happening. The numbers aren't there.

There are six states with Democratic legislatures and Democratic governors that haven't yet passed the deal: Oregon, Minnesota, Delaware, Connecticut, Colorado and West Virginia. There's also New York, where Republicans sort of control the Senate but love the idea, and where the main bottleneck has been the opposition of the Democratic State House speaker, Sheldon Silver. If Silver can be persuaded, and all six Democrat-controlled states also adopt the pact, that gets the deal to 206 electoral votes. That's 76.3 percent of the way there. But that last 23.7 percent is going to be really tough.

Here are a few ways to get those final 64 electoral votes, all of them tough:

-- Texas and Florida alone would bring 67 electoral votes. That's enough right there. But Texas and Florida both have Republican-controlled legislatures and Republican governors. Florida's governorship could very well flip hands, but flipping both the state House and Senate is a tall order. So unless Republicans suddenly get into electoral college reform, those two are probably out of contention.

-- Maine, Nevada, and New Mexico all have Republican governors but Democratic legislatures. But to get them, Democrats would have to knock off popular Republicans like Susana Martinez, or else win super-majorities in state legislatures. That's not easy. And even if they got all three, you're only halfway there.

-- Arkansas, Kentucky, Missouri, Montana and New Hampshire all have Democratic governors but Republican legislatures. That's 31 electoral votes. But getting them means the governors have to play hardball with their legislatures, perhaps sacrificing some other priorities, and even that might not work. Ultimately, they can't force the change through. And again, 31 isn't enough. Thirty-one plus the 29 from states with Republican governors but Democratic legislatures isn't even enough.

All of which is to say that one of two things needs to happen if the National Popular Vote is going to come into effect. Either Democrats need to win a lot — a lot — of state elections, or else the issue has become nonpartisan and Republican legislators in deep red states have to get on board.

You could imagine a reverse-2000 scenario, where a Democrat wins in the electoral college but loses the popular vote and Republicans suddenly begin wondering whether the college is such a good idea after all. Somewhat less plausibly, you could imagine, for instance, Texas Republicans getting tired of being ignored in national presidential races just because the state is reliably red, and signing on. That would be a big shift from the trends to date, but reformers are going to need something big for this to happen.