Last week, President Obama made it clear that he will soon offer some form of limited amnesty to about five million foreign nationals who are currently living illegally in the United States. He will do so by issuing an executive order to federal officials who oversee immigration directing them to undertake a course of action that, if complied with individually by all persons whom he designates as eligible, will cause the federal government to remove the threat of deportation from those who meet the standards he will lay down.

Can he legally do that?

To address that question, we need to start with the principle that a presidential action may be lawful at the same time that it is unconstitutional. The president has the legal power to defer deportations. The power is called prosecutorial discretion. This is a power traditionally recognized as inherent in the presidency that enables him to defer or modify all federal law enforcement.

Too Much StuffThe theory is that the president needs the ability to allocate resources as the changing times, emergent events and public needs may require. Thus, he can, for example, defer prosecuting bank robbers and aggressively pursue drug dealers. That wouldn’t mean that all bank robbers would go free; it would mean that either state prosecutors would pursue them, or they’d wait for trials until the drug kingpins were caught and convicted. But he could set some free if he wished.

The check on the exercise of prosecutorial discretion is gross abuse, which is typically demonstrated by either improper executive motive or effective nullification of law. I don’t know what the president’s motive is. If it is political, I suspect his efforts will backfire. He cannot grant citizenship or the right to vote.

If his motive is humanitarian or moral, I understand him. Under the natural law, people have the right to travel and live wherever they wish. The existence of our natural rights is not conditioned upon the place where our mothers were at the times of our births. And from a free market and historical perspective, immigrants have enhanced the economy as they move up the demographic ladder.

But the president’s behavior has serious constitutional dimensions that go far beyond the motives in his heart, and his oath is to the Constitution, not to his heart.

If the president nullifies deportations on such a grand scale that the effect is the nullification of federal laws, then he has violated his oath “faithfully” to execute his presidential obligations.

The Framers required that every president swear to do his job “faithfully” to serve as a reminder to him that his job requires fidelity to the enforcement of laws with which he may disagree. The American people, Congress and the courts need to know we have a president who will enforce the laws, whether he agrees with them in his heart or not. Without presidential fidelity to the Rule of Law, we have a king, not a president.

By conferring temporary legal status upon foreign nationals who have not achieved it under the law, providing they meet criteria that he will establish, the president affects huge numbers of persons and produces a result that is the opposite of what the law requires. Can the president’s exercise of his prosecutorial discretion constitutionally nullify a federal statute? No. Can the president’s exercise of his prosecutorial discretion effectively rewrite a federal statute? No.

It is unconstitutional for the president to nullify federal law. It is unconstitutional for him to refuse to enforce laws that affect millions of persons and billions of dollars. It is unconstitutional for him to refuse to enforce laws merely because he disagrees with them — particularly laws that pre-existed his presidential oaths. And it is unconstitutional for him to rewrite laws, even if he is doing so to make them more just.

Every president since Dwight D. Eisenhower has deferred some deportations. President Reagan deferred deportations for about 100,000 families of foreign nationals in 1987 under his reading of the congressionally authorized 1986 amnesty law, and President George H.W. Bush did so in 1990 for about 350,000 foreign nationals under his reading of the same law. Each of these was based on a principled public presidential reading of the words and purposes of a federal statute. Obama does not purport to read and interpret the current immigration law; rather, he effectively rewrites it.

What can Congress do? Congress can pass legislation to invalidate Obama’s executive actions. Yet even if it did so and overrode his certain veto, it has no assurances that Obama would be bound by the new legislation. He refuses to enforce the plain language of well-established and never judicially altered federal statutes. What assurances does Congress have that he would follow any new statutes that he has vetoed and that regulate his behavior?

Is the blanket refusal to enforce federal laws that profoundly affect five million persons — and in the process severely straining the social services of all 50 states — an impeachable offense? The president is playing with constitutional fire, and impeachment is the only constitutional remedy available, short of 25 months of a constitutional conflagration that he has ignited.

Andrew P. Napolitano, a former judge of the Superior Court of New Jersey, is the senior judicial analyst at Fox News Channel. Judge Napolitano has written seven books on the U.S. Constitution. The most recent is Theodore and Woodrow: How Two American Presidents Destroyed Constitutional Freedom. To find out more about Judge Napolitano and to read features by other Creators Syndicate writers and cartoonists, visit creators.com.