UPDATE (Aug. 29, 2019, 2:33 p.m.): On Thursday, the Colorado secretary of state certified that the effort to put Colorado’s National Popular Vote law to a public vote had indeed gathered enough signatures to do so. The referendum will now appear on the 2020 ballot.

In March, the state of Colorado handed a historic win to opponents of the Electoral College by becoming the first purple state to sign on to the National Popular Vote interstate compact. Next November, however, it could make history yet again by becoming the first state to renege on the agreement.

As we’ve written previously, states that join the National Popular Vote compact agree to cast their electoral votes for the presidential candidate who wins the most votes nationwide — not necessarily the candidate who carries the state. And the compact only goes into effect once states worth 270 electoral votes (a majority in the Electoral College) have joined, thus ensuring that its signatories have enough electoral votes to guarantee that the national popular vote winner becomes president. Currently, 15 states plus the District of Columbia, together worth 196 electoral votes, have ratified the compact.

Four of those states, including Colorado, joined the National Popular Vote movement just this year, but it remains a controversial issue — for example, it recently failed to pass in Maine and was vetoed in Nevada. And opponents in Colorado were upset enough about its passage that they are now actively trying to repeal the law. Earlier this month, the organization Coloradans Vote said it submitted more than 227,198 signatures to the Colorado secretary of state in an effort to subject the law to voter referendum in the 2020 election. With that number of signatures, chances are very good it will make the ballot, making it the first time voters in any state will vote directly on the National Popular Vote compact.

According to the Colorado secretary of state’s office, the 227,198 signatures are likely the most ever submitted for a statewide ballot initiative in Colorado — certainly the most since at least 2001. Now, it’s typical for about 20 percent of signatures to be thrown out during the verification process. But because the referendum needs only 124,632 valid signatures to qualify, up to 45 percent of them could be tossed and the measure would still make the ballot. (The secretary of state’s office will announce whether it has done so by Aug. 30.)

So the real question becomes whether voters will reject the legislature’s law and make Colorado the first state to exit the National Popular Vote compact. And polls suggest it would be a competitive election! Nationally, 53 percent of Americans said the popular vote should determine the president, and 43 percent said the Electoral College should, according to an April/May NBC News/Wall Street Journal poll. Unsurprisingly, given that almost every state government to pass the National Popular Vote compact was completely controlled by Democrats, there is a wide partisan gap on the question: 79 percent of Democrats preferred the popular vote, while 74 percent of Republicans favored the Electoral College.

Given that Colorado is roughly evenly divided between Democrats and Republicans, I’d expect support to break down about the same in the Centennial State as it does nationally. The only poll about the National Popular Vote law I could find in Colorado was a March survey from Republican pollster Magellan Strategies that found 47 percent of likely 2020 voters would vote to affirm the National Popular Vote law and 47 percent would vote to repeal it. However, even if those numbers are too rosy for the repeal effort, I would still expect support for the law to decrease as opponents prosecute the case against the National Popular Vote, so even a lead of, say, 10 points (akin to the national breakdown) would not be secure. This could be one of the most closely watched ballot measures of the 2020 cycle.

CORRECTION (Aug. 16, 2019, 10:50 a.m.): A previous version of this article incorrectly characterized the National Popular Vote compact referendum as likely to make the 2020 ballot in Colorado. If the referendum passes, it would not repeal the National Popular Vote law. The referendum campaign would put the law itself up for a vote, so Colorado would have to vote “no” on the referendum in order to exit the compact.