This month’s debates between President Obama and Mitt Romney are focusing on the central position of the presidency in our political system.The president is the leader of his political party, his views on the issues of the day come first, even in areas where he has little power, and he is the center of the 24-hour news cycle.

But what is equally important is the role of the president set out in the Constitution. Although it might not come up in the debates, Obama has pursued a dangerous change in the powers of his office that disregards the Constitution’s careful separation of power between the branches of the federal government. The Constitution imposes on the president two clear duties – to protect the national security and to “take Care that the Laws be faithfully executed.” Obama is the first chief executive since Richard Nixon to ignore a duly-enacted law simply because he disagrees with it, in clear defiance of his constitutional duty.

Obama put his radical vision of executive power clearly on display this summer when he announced that he would refuse to deport up to a million illegal aliens, as required by the immigration laws. According to the Department of Homeland Security, they will not be deported if they came to the United States under age 16 and are currently under 30, and have not committed any major crimes. Even though Congress has failed to pass the DREAM Act, which would create such a program, Obama has commanded DHS to grant these aliens work permits for periods of up to two years.

“It makes no sense to expel talented young people who for all intents and purposes are Americans,” the president said at a Rose Garden press conference.

According to Homeland Security Secretary Janet Napolitano, “these young people do not present a risk to safety or security.”

Obama’s order has pushed the executive power beyond all constitutional limits. I have long been an academic defender of a vigorous presidency. As a Justice Department lawyer in the Bush administration, I took the view that the White House could refuse to carry out an unconstitutional law that infringed on the president’s commander-in-chief authority to manage war and defend the national security. I agree that our immigration system demands fundamental reform, particularly in how it treats those brought here as infants.

But the president cannot refuse to enforce a law simply because he disagrees with it. In limited circumstances, a chief executive can refuse to follow a law that is itself unconstitutional. But Article I, Section 8 of the Constitution gives Congress, not the president, the authority “to establish a uniform rule of naturalization.” Although the Constitution is silent on border control and immigration, the Supreme Court declared long ago that these authorities reside with Congress. Congress has passed an extensive Immigration and Naturalization Act, which specifies the limited cases where the executive branch can suspend the removal of illegal aliens. The act does not give the president the authority to interrupt the deportation of whole classes of illegal aliens, and certainly nothing approaching one million.

Worried about Hispanic support for his re-election, however, Obama simply decided to unilaterally enact his own legislation. But under Article II, Section 3 of the Constitution, the president has the duty to “take Care that the Laws be faithfully executed.” Since the days of Machiavelli, through Hobbes, Locke, and Montesquieu to the Framers, execution of the laws has formed the very core of the executive power. As Alexander Hamilton explained in Federalist 75: “The execution of the laws and the employment of the common strength, either for this purpose or for the common defense, seem to comprise all the functions of the executive magistrate.”

The only exception, other than an unconstitutional law, to the president’s duty to carry out federal laws is prosecutorial discretion. Discretion recognizes that limited time and resources prevent prosecutors from pursuing every violation of federal law. The Justice Department must choose priorities and prosecute cases that caused the most harm, have the greatest impact, and deter the most dangerous criminals. But Obama cannot deploy discretion to rewrite a federal law, especially when he is enforcing the rest of immigration law. Otherwise, President Mitt Romney will repeal ObamaCare by refusing to fine or prosecute health insurers and consumers who disobey federal regulations. He will lower tax rates simply by declining to prosecute anyone who doesn’t pay his full taxes. Allowing presidents to pick-and-choose which laws to enforce effectively gives him, and not Congress, the legislative power granted by the Constitution to the federal government.

Obama’s supporters, of course, may well argue that Obama’s immigration proclamation is no worse than President Bush’s claim that Congress cannot limit the executive’s efforts to intercept Al Qaeda communications during wartime. But there is a constitutional world of difference in refusing to enforce laws that violate the president’s own constitutional powers, and ignoring laws that a president simply dislikes. There is a world of difference between putting aside laws that interfere with an executive response to an attack on the country, as in Sept. 11, 2001, and ignoring laws to appeal to a constituency vital to re-election. The former recognizes the president’s primary duty to protect the national security. The latter, unfortunately, represents a twisting of the Constitution’s fabric for partisan ends.

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John Yoo, who served in the Bush Justice Department from 2001-03, is a law professor at the University of California at Berkeley and a visiting scholar at the American Enterprise Institute. He is the author most recently of Taming Globalization: International Law, the U.S. Constitution, and the New World Order (Oxford 2012).