This story was first published May 28, 2019. It has been updated.

(CNN) Why bother amending the Constitution when you can just agree to ignore it?

That's essentially the idea behind the National Popular Vote Interstate Compact , whereby states agree to pledge their electoral votes to whichever candidate wins the popular vote nationwide.

In an alternate popular vote universe, you'd have had President Al Gore. Or President Hillary Clinton . Which is why Democrats, who are concentrated in cities and in coastal states, tend to really love the idea.

The compact can't take effect until it's joined by states representing 270 electoral votes, the magic number it takes to elect a president. It's been gaining traction, though so far only states that went for Clinton in 2016 have signed on.

If Maine's Democratic governor goes the other way and signs on, that will mean 15 states plus Washington, DC, will have agreed to pledge their 193 electoral votes to the popular vote winner. That's 65 electoral votes shy of the 270 they need for the compact to become active.

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The proposal has passed one chamber in a handful more states, including Republican-controlled legislatures in Oklahoma and Arizona.

That's a key point. The idea that the candidate who gets the most votes should win is not that crazy in a country that considers itself a democracy, but getting there will require states that voted for President Donald Trump in 2016 to sign on.

And that will mean convincing Republicans, who control almost two-thirds of state legislatures and a smaller majority of governorships, to give up the institutional advantage that has given them multiple recent presidents who got fewer votes than the also-rans.

What does the Constitution say?

The Constitution requires that electors pick the President, but it doesn't say too much about how the electors are chosen.

Early in US history, many state legislators did the job, mostly cutting individual voters out of the process. The system of states bundling their electoral votes into winner-take-all packages evolved over time, but it has been the norm since the 1800s.

Some states still have their own rules. Maine and Nebraska award their electoral votes proportionally, which means that more than one candidate can emerge from an election with electoral votes. That's how Trump got an electoral vote from Maine in 2016 and Barack Obama got one from otherwise red Nebraska in 2008

There's nothing in the Constitution or federal law that requires electors to vote for the candidates chosen by the voters in their states. It's an honor system. In 2016, seven so-called "faithless" electors went their own way , breaking with their state results.

But changing the Constitution is a hard process that requires multiple steps and super-majority support.

That's why the interstate compact seems to its supporters like an elegant and much simpler solution. The Electoral College would still meet and select the President under this system, so just as the state-based winner-take all system grew up in the 1800s , perhaps a pact to select the popular vote winner is just the next step in an evolutionary process.

But that's also what lets opponents argue the opposite: that it's a way for Democrats to undermine the system -- in their own favor.

The problem with competing majorities

It is a fact of American politics that demographic trends favor Democrats, who run strong in cities and population centers, while the Electoral College tends to favor Republicans.

As the pollster Gary Langer said recently, growth in urban areas won't necessarily help Democrats defeat Trump in 2020. It just means more disappointed people, clustered together.

"It's clear regardless that the Democrats' over-vote in urban areas leaves them more vulnerable outside the big cities than their aggregate support would suggest," Langer wrote in a recent paper

at least since 2000. The last few election cycles have seen the rise of countermeasures like the Democrats have been frustrated by the Electoral Collegeat least since 2000. The last few election cycles have seen the rise of countermeasures like the "Great Schlep" of 2008 -- a political stunt to get blue-state transplants to evangelize their parents or grandparents in Florida and other swing states on behalf of Obama.

Several 2020 Democratic presidential candidates have already called for the Electoral College to be abolished, notably Sen. Elizabeth Warren of Massachusetts.

"My view is that every vote matters, and the way we can make that happen is that we can have national voting, and that means get rid of the Electoral College -- and every vote counts," she said at a CNN town hall earlier this year.

The interstate compact does not go as far as abolishing the Electoral College, but it would effectively neutralize its power.

Arguments against the compact

There is some notable opposition to the idea. Former Maine Republican Gov. Paul LePage said this year that under the pact, "white people will not have anything to say."

"What would happen if they do what they say they're going to do, white people will not have anything to say," LePage said on WVOM radio. "It's only going to be the minorities that would elect. It would be California, Texas, Florida."

That race-based opposition is ugly and unfortunate, and LePage's Democratic successor is set to add Maine to the compact.

But Trump has said on Twitter that ending the Electoral College would change the way US elections are run.

"Campaigning for the Popular Vote is much easier & different than campaigning for the Electoral College. It's like training for the 100 yard dash vs. a marathon. The brilliance of the Electoral College is that you must go to many States to win. With the Popular Vote, you go to....," he tweeted in March, "....just the large States - the Cities would end up running the Country. Smaller States & the entire Midwest would end up losing all power - & we can't let that happen. I used to like the idea of the Popular Vote, but now realize the Electoral College is far better for the U.S.A."

Campaigning for the Popular Vote is much easier & different than campaigning for the Electoral College. It's like training for the 100 yard dash vs. a marathon. The brilliance of the Electoral College is that you must go to many States to win. With the Popular Vote, you go to.... — Donald J. Trump (@realDonaldTrump) March 20, 2019

....just the large States - the Cities would end up running the Country. Smaller States & the entire Midwest would end up losing all power - & we can't let that happen. I used to like the idea of the Popular Vote, but now realize the Electoral College is far better for the U.S.A. — Donald J. Trump (@realDonaldTrump) March 20, 2019

It's only natural for a popular-vote-losing President to have that view. Supporters of the compact argue that under the current system candidates focus only on a few winnable swing states, which has the effect of the presidential election really being fought only in a handful of places like Florida, the Rust Belt, North Carolina and Arizona.

But if the pact were enacted, you can imagine the Supreme Court would have to weigh in. It would be half the states joining together to change the political process for everyone. There is no prohibition on interstate compacts , but Congress might ultimately have to weigh in.

Others have pointed out that a nationwide recount in case of an extremely close election would be extremely difficult and expensive. The 2000 statewide recount in Florida was a national event. Imagine that on a national scale.

There have been extremely close elections. In 1880, James Garfield won the popular vote by fewer than 2,000 ballots. Richard Nixon lost to John Kennedy by about 112,000. Situations like this could test the compact, as could an election with multiple candidates where the person with the most popular votes was still far short of 50%. However, several US presidents, including Bill Clinton, were elected this way.

It is ultimately an academic conversation, however, unless and until Republicans agree to cede their current Electoral College advantage and buy into the idea of elevating the popular vote.