[On Thursday, constitutional scholars Ilya Shapiro and Ilya Somin will debate the constitutionality of President Obama's executive order immigration in an event at Reason's Washington, DC office moderated by Reason Foundation Senior Analyst Shikha Dalmia. Both scholars are previewing their arguments at Reason.com today. Read Shapiro's case against the constitutionality of Obama's order here.]

The Obama administration’s policy of deferring deportation for over four million undocumented immigrants is the focus of United States v. Texas, a major case currently before the Supreme Court, in which numerous state governments challenge the president’s order. While Obama’s action highlights the alarming extent of executive discretion in the modern state, it is not unconstitutional.

Whitehouse.gov

The most fundamental reason why Obama’s policy is legal is one that has not been raised in the litigation over it: The immigration laws whose enforcement it restricts are themselves unconstitutional. The detailed list of congressional powers in Article I of the Constitution does not include any general power to restrict migration.

The Naturalization Clause gives Congress the power to establish a "uniform Rule of Naturalization." But it does not grant any authority over migration, as opposed to eligibility for citizenship. The power to regulate "commerce with foreign Nations" is another possible source of congressional power. But during the Founding era and for many decades thereafter, the dominant interpretation was that it merely gave Congress the power to restrict trade and other commercial transactions, not to forbid movement. The Commerce Clause also gives Congress the power to regulate interstate as well as international commerce. Yet almost no one at the time of the Founding believed that Congress therefore had the power to forbid Americans from moving from one state to another.

Congress can restrict the entry of some foreigners by using its other enumerated powers. For example, Congress’ war powers enable it to ban entry by enemy spies, terrorists, and soldiers. But there is no general congressional authority to ban the entry of people simply because they are foreign nationals.

Not until the Chinese Exclusion Act of 1882 did Congress adopt a significant law banning migration as such, as opposed to narrowly defined categories of migrants who might fall within the scope of other enumerated powers. It forbade nearly all immigration from China, and was largely motivated by racist hostility to the Chinese. In 1889, the Supreme Court upheld that law on the basis of crude, nonoriginalist reasoning holding that the power to restrict migration was "inherent" and did not need to be enumerated—even though the framers did enumerate such far more fundamental powers as the authority to declare war and raise armies.

Adherents of "living constitution" theories of interpretation can endorse this result. But originalists—including most of the critics of Obama’s action—cannot.

This is especially true for those conservative originalists who have argued for a narrow interpretation of Congress’ powers under interstate Commerce Clause. If the term "commerce" has a narrow definition when it comes to interstate commerce, the same applies to foreign commerce. The Constitution literally uses the same word to cover both, giving Congress the power to "regulate Commerce with foreign Nations, and among the several States."

Obama’s policy is constitutional even aside from the dubious nature of the immigration restrictions it affects. Most of his order is simply an exercise of longstanding executive discretion in deciding when to enforce federal laws. There are more than 11 million illegal immigrants in the United States, and no administration is likely to deport more than a fraction of them.  A majority of Americans have violated federal criminal law at some point in their lives. Only a small fraction are ever prosecuted. The executive has  unavoidably broad discretion to decide which suspected lawbreakers to pursue and which to ignore.

This is far from the only case where federal law enforcement agencies have chosen to systematically ignore a large category of lawbreakers. Federal prosecutors virtually never target marijuana possession by students on college campuses, even though it is widespread and ubiquitous. The number of people—including the last three presidents of the United States—who have gotten away with violating federal law in this way greatly exceeds the number exempted from deportation by Obama’s policy.

Lower court rulings against the Obama policy hold that it goes beyond ordinary executive discretion because it replaces "case by case" discretion with categorical rules imposed by the president. But the difference between case by case examination and categorical rules is one of degree, not kind. Unless case by case discretion is completely arbitrary, it must be guided by some sort of generalizable criteria, such as the severity of the offense. And if general rules like this can be applied by low-level law enforcement offenders handling particular cases, they can also be applied systematically by the president, their ultimate superior.

This consideration should be especially salient to conservative originalist advocates of the "unitary executive" theory, which holds that the Constitution requires all executive branch officials to be under the control of the president. Given the vast scope of modern federal law enforcement, often the only effective way for the president to exercise effective control is to issue general rules that bind his subordinates. At yesterday’s Supreme Court oral argument, Texas Solicitor General Scott Keller actually admitted that the administration can systematically defer deportation, even for millions of people, so long as "that’s all they are doing."

Critics also claim that Obama has violated the Take Care Clause of the Constitution, which requires the president to "take care that the laws are faithfully executed." But that Clause surely does not require the president to enforce every federal law to the hilt, especially in a world where it would be literally impossible to even come close to doing so. Otherwise, virtually every president would be in constant violation of the Clause.

Historical evidence suggests that the Clause was meant to prevent "suspension" of the laws—a presidential determination that an otherwise illegal action is lawful. Obama’s order does no such thing. It does not legalize any otherwise illegal immigration any more than failure to prosecute marijuana possession on college campuses makes it legal to get high in a dorm room. It merely states that the administration will not deport certain categories of (admittedly illegal) aliens until such time as the president may decide otherwise.

The plaintiffs are on stronger ground in emphasizing that the new policy goes beyond deferring deportation because it allows many of the covered aliens to be legally employed in the US, which is otherwise forbidden by federal law. But this part of the policy is explicitly authorized by the 1986 Immigration Reform and Control Act, which specifically permits employment of aliens who are "authorized…to be employed…by the attorney general."

Much attention has recently focused on the grant of "lawful presence" to the immigrants covered by the order. While this may seem like a big deal, in reality "lawful presence" does not actually legalize the presence of any otherwise illegal migrants, and does not prevent any from being deported at any time the president might choose. For the most part, it merely reiterates the executive’s discretionary decision not to deport the migrants covered by the order. It does, however, also allow them to accrue time for the receipt of Social Security and Medicare benefits that, however, they are unlikely to ever actually collect unless their status is genuinely legalized at some point in the future, and they remain in the US until after retirement age. Moreover, the "lawful presence" element of the President’s order can easily be struck down separately, without affecting the other, far more important aspects of the policy.

Obama’s exercise of discretion in this area highlights the enormous power enjoyed by the modern executive in a world where nearly everyone is guilty of violating some federal law. Like undocumented migrants who manage to avoid deportation, many of us are also free only as long as officials choose to overlook our violations of labyrinthine federal laws. This reality creates numerous opportunities to abuse executive power by means of selective enforcement.

The way to address that danger, however, is not to attack Obama’s effort to protect immigrants, but to work to reduce the enormous scope of federal law that created such vast discretion in the first place.

Ilya Somin is professor of law at George Mason University. He is the author of Democracy and Political Ignorance: Why Smaller Government is Smarter, and The Grasping Hand: Kelo v. City of New London and the Limits of Eminent Domain. Parts of this column are adapted from earlier blog posts at the ACS Blog, and the Open Borders website.