The Circuit Court of Cook County in Chicago has agreed to hear a lawsuit on Sen. Ted Cruz's eligibility for president — virtually ensuring that the issue dominates the news in the runup to the South Carolina primary.

Cruz was born in Canada to a US citizen mother and a noncitizen father. The Constitution requires presidents be "natural-born citizens," but what exactly that requires hasn't been settled in court.

Now, perhaps, it will be. The lawsuit in Illinois aims to resolve the question by challenging Cruz's eligibility for the presidency. It was filed by Lawrence Joyce, an attorney who has told local media that he supports Dr. Ben Carson and has had no connection with the Trump campaign.

"Joyce said his concern is that the eligibility issue lie unresolved during Republican primaries, thus letting the Democrats take advantage of it after a potential Cruz nomination, when it’d be too late," reports the Washington Examiner.

When this question initially came up, the conventional wisdom among constitutional lawyers was that it was a nonissue: Cruz was obviously eligible. But as the debate has heated up among candidates (with Donald Trump, in particular, fanning the flames), it's also begun to heat up among constitutional law scholars.

The issue is actually twofold: whether Ted Cruz should be considered a natural-born citizen, and whether Cruz's own preferred school of constitutional interpretation would see it that way.

The problem: the meaning of "natural-born citizen"

Here is what the Constitution says about who can be president:

No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.

The problem is the Constitution doesn't define "natural born Citizen." Neither does any current law. And no one has ever brought a court case to decisively settle the question as a matter of US law.

There are three ways someone can be a US citizen. He can be born in the US (regardless of who his parents are). He can be born outside the US to at least one US citizen parent, as long as certain criteria are met. (Those criteria are set by federal law and have been changed over time.) Or he can immigrate here and then successfully apply for citizenship, a process called naturalization.

Everyone agrees that the first category of people are natural-born citizens. Everyone agrees that the third category of people are not natural-born citizens (regardless of how unfair it might be that immigrants can't be president). But Ted Cruz is in the middle category, and this is where the meaning of "natural born" starts to get fuzzy.

The only definition of "natural born" in US history would include Ted Cruz

Because there's never been a court case to explicitly test the question of who counts as a natural-born citizen for the purpose of presidential eligibility, the question is by definition "unsettled." It hasn't been resolved yet. And court opinions that have mentioned the term in passing while ruling on other questions have come to very different opinions about what it means.

But it's a stretch to say, as Harvard law professor Laurence Tribe did, that the scholarship on the question is "completely unsettled." That implies that scholars are totally split on the issue, which isn't exactly the case.

The majority of constitutional law scholars who've written about the meaning of "natural-born citizen" have agreed that if a court were to rule on the question, it ought to rule that someone born outside the US but eligible for citizenship through parents counts as "natural born."

One of the key arguments in favor of this point is that while there is no longer any law defining "natural born," there used to be one — way back in 1790. The Naturalization Act of 1790 explicitly said that "the children of citizens of the United States, that may be born beyond sea, or out of the limits of the United States, shall be considered as natural-born citizens."

That term disappeared from immigration law after 1795. While there's at least one scholar who argues that this was intentional, because Congress didn't want that definition to persist, there's no evidence for that. And since Congress didn't come up with an alternate definition, that remains, to this day, the only definition of "natural born" we have.

This isn't a smoking gun. Scholars have looked at English precedents, US judicial decisions, bills, and congressional debates to figure out what the meaning of "natural born" is supposed to be and how (if at all) it's changed over time. But while some scholars have maintained that the evidence supports a narrow meaning of "natural born" — one that wouldn't include Ted Cruz — more of them agree that the evidence supports a broader one.

What would legal scholar Ted Cruz say about the eligibility of candidate Ted Cruz?

One of the constitutional scholars who used to think that the definition of "natural born" ought to include Ted Cruz is Laurence Tribe, who was Cruz's law professor at Harvard. But Tribe is now the leading scholar raising questions about Cruz's eligibility. Trump has taken to citing Tribe approvingly in rallies; Cruz has fired back that Tribe is a liberal professor who is only interested in taking him down.

Why is Tribe raising questions about Cruz's eligibility, even if Tribe thinks Cruz should ultimately be eligible? There are two answers.

The first answer is that Tribe is making a claim about what Ted Cruz ought to believe the Constitution says.

Cruz is a proud supporter of the conservative legal tradition of constitutional originalism: interpreting the Constitution not by what its words ought to mean today, but by what the Founding Fathers meant as they wrote them in 1787. Cruz is arguably the national politician most closely identified with originalism; he's certainly the presidential candidate with the closest ties to the conservative legal movement.

According to Tribe, constitutional originalism defines "natural born" very narrowly, in a way that would exclude Cruz. By extension, Tribe argued in the Boston Globe, any judges Cruz would appoint to the federal bench as president would invalidate his own presidency.

But Tribe clearly doesn't believe this line of argument himself because he is very much not an originalist. And one of the points of his column is that maybe if originalism is such an inflexible theory that it wouldn't allow one of its own biggest supporters to be president, it is generally a bad idea. He points out that the reason the Founding Fathers didn't want immigrants to be president is totally moot today — but so is the idea of a "well-ordered militia." And if originalists like Cruz still support the Second Amendment, Tribe says, they can't wave away the "natural-born citizen" clause.

Originalists disagree about what originalism is and what it says about "natural born"

While you wouldn't know it from Tribe's piece, there is no one originalist take on what "natural-born citizen" means. The strongest supporters of a narrow definition that would exclude Cruz are generally originalists, but there's a more even split among originalists than there is among constitutional scholars as a whole.

Since the Founding Fathers never actually debated the meaning of "natural-born citizen" when writing the Constitution, originalist scholars have had to turn to other sources to figure out what the common understanding of the phrase would have been at that time. And the answers scholars come to differ depending on which sources they consult.

Some originalists, like Michael Ramsey of the University of San Diego — who fortuitously just finished a paper on this question when the topic came up in the campaign — argue that the Founding Fathers would have understood "natural-born citizen" to mean the same thing "natural-born subject" did in English law at the time.

Over the century before the Revolution, Parliament had passed several bills clarifying that children born abroad to British subjects counted as "natural-born subjects" (this mattered for inheritance reasons). So by the time the Founding Fathers were writing down the Constitution, the broad definition of the term was fairly well established.

Other originalists, like Mary Brigid McManamon of Widener University's Delaware Law School — who recently published a column in the Washington Post arguing that Cruz is ineligible to be president — think that laws passed by Parliament don't count.

To McManamon, the precedent the Founding Fathers used wasn't British law as of 1787, but the English common law tradition (law made by courts rather than legislation). And in the common law, "natural born" did not apply to children born outside the bounds of the country. That's why Parliament had to pass bills to include such children.

Each of these arguments is far more complicated, of course. (For one thing, some scholars argue that the common law wasn't as uniformly narrow as McManamon says it was.) But the debate among originalists as to what "natural born" means is really a debate among originalists as to what originalism ought to include. Should it include both common law and legislation, or just common law? Does a law passed in 1790 reflect the intent of the Founding Fathers, since so many of them were in Congress when it passed, or does it show that they needed to add something they thought wasn't in the Constitution already?

The truth is that there isn't nearly as much of a gulf between originalism and "living constitutionalism" as there might seem to be. Originalists look to a number of sources to figure out what the Constitution means, just like anyone else does. And even the living constitutionalists who've written about natural-born citizenship care about what the Founding Fathers meant it to mean at the time — that's just not the be-all and end-all of their jurisprudence.

This can only be settled in court. But who would nominate a walking court case?

Ultimately, this is, quite literally, an academic debate. As long as no US court has issued a ruling on the question, it wouldn't matter if every legal scholar in America agreed on the hypothetical meaning of "natural born." It would still be legally unsettled.

Congress could at least stick some kind of bandage on the question by passing a "sense of the Congress" resolution — that's what it did in 2008 to affirm the eligibility of John McCain, who landed in the "natural born" gray zone for different reasons from Cruz. But the Senate has made it clear that it intends to do no such thing for Ted Cruz. This probably is less because they don't think Cruz is natural-born than because Senate Republicans really don't like Ted Cruz, but it's a problem for him nonetheless.

That's the other answer to why Tribe is agitating against Ted Cruz. He doesn't believe any court in the country would actually rule that Cruz was ineligible (though, he claims, that's only because Cruz-style originalism isn't the norm). But, he writes, "it’s worth thinking about the legal cloud" hovering over Cruz in the meantime.

The problem for Ted Cruz here isn't so much that a court is likely to rule against him as it is that Republicans might be afraid to support Cruz for the nomination because they're worried his eligibility will become an issue. A court taking up the issue days before the South Carolina primary is pretty much his worst nightmare.