The winner-take-all presidential system is unconstitutional. We're taking it to court.

In the Electoral College, winner-take-all is the name of the game in nearly every state. It's why swing states are so critical: Winning a state by just a few thousand votes can lead to a huge haul of electoral votes, but losing by just a few means going away empty-handed. So winner-take-all leads to millions of votes being tossed out, and to a question few have considered until recently: Is the winner-take-all allocation of Electoral College votes even legal?

Up to now, most people have thought of winner-take-all as just the weird, quirky way we elect presidents in our country. America’s done it this way for centuries, they think. This is the system required by the Constitution, they might even think. So of course it’s legal, they conclude.

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But longstanding practice or tradition doesn’t by itself make something legal. The Supreme Court famously reaffirmed this in its recent decision that states could not continue to define marriage as between one man and one woman just because that’s the way it had always been defined.

Moreover, the winner-take-all system is not required by the Constitution. Instead, the Constitution leaves the choice of allocation of presidential electors up to each state, but that choice is subject to the “one person, one vote” requirement of the Equal Protection Clause of the Constitution.

My organization, Equal Citizens, is working with a bipartisan team of lawyers that filed legal challenges to the winner-take-all system last month in four states — California, Massachusetts, Texas and South Carolina. Equal Citizens also ran a crowd-funding campaign to support the project and developed the legal theories underlying the lawsuits.

The challenge for winner-take-all’s defenders is serious. If we take away the “we’ve always done it that way” argument (doesn’t matter legally), the “it’s in the Constitution” argument (winner-take-all’s not in there) and the “states can choose presidential electors however they want” argument (they can’t, because they need to comply with “one person, one vote”), then the foundation for our current system starts to look less stable. In fact, our system crumbles.

It turns out there is no argument that can legally justify states’ use of winner-take-all to allocate electors. It’s an unconstitutional way to pick the presidential electors who ultimately cast votes for president and vice president.

That conclusion may seem stunning, but it’s actually a straightforward application of a principle of fairness and equality of citizenship that has been in our Constitution since the Equal Protection Clause was added following the Civil War.

To see why, it’s important to remember that voters don’t actually cast votes for president but instead vote for slates of presidential electors expected to support that candidate with a vote in the Electoral College. For example, in 2016, California was entitled to choose 55 presidential electors, and, because Hillary Clinton got more votes than any other candidate in the state, California used winner-take-all to select 55 Democratic presidential electors who would all cast votes for Clinton for president.

Donald Trump managed to get over 4.5 million votes in California, more than in any state he won except for Texas and Florida, but they were tossed out. Trump’s 4.5 million popular votes translated into zero presidential electors.

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This violates the Constitution’s principle that all votes matter equally. Imagine if California got rid of its 40 state Senate districts and instead asked voters to simply vote for a Democratic or Republican majority leader. And then imagine that instead of allocating the 40 senators to match the two parties' share of the vote, the state gave the winning party all 40 state Senate seats. So the Senate would always be 40-0 in favor of one party.

This winner-take-all scheme for electing state senators would be illegal, because it violates one person, one vote. A state can’t turn a contested election for multiple seats into a 40-0 blowout. This principle applies in selecting presidential electors as much as it does for state senators.

Some might dismiss the analogy by saying that the winner-take-all state Senate scenario is a ridiculous hypothetical. After all, no state has anything like it, and it seems absurd. But that’s exactly the point: Using a winner-take-all system to choose the members of a multi-member body is so plainly unfair and unconstitutional that no state would dream of implementing it for most of its state offices. And yet 48 states currently have just such a system for selecting presidential electors. It must be put to an end.

Jason Harrow is chief counsel for Equal Citizens, a non-profit working to end states’ use of winner-take-all in the Electoral College.