There are two exceptions, neither of which applies here. The first is that "the Laws" includes the Constitution. The president can and should refuse to execute congressional statutes that violate the Constitution, because the Constitution is the highest form of law. We in the Bush administration argued that the president could refuse to execute laws that infringed on the executive's constitutional powers, particularly when it came to national security - otherwise, a Congress that had a different view of foreign policy could order the military to refuse to carry out the president's orders as Commander-in-Chief, for example. When presidents such as Jefferson, Jackson, Lincoln, and FDR said that they would not enforce a law, they did so when the law violated their executive powers under the Constitution or the individual rights of citizens.

The president's right to refuse to enforce unconstitutional legislation, of course, does not apply here. No one can claim with a straight face that the immigration laws here violate the Constitution.

The second exception is prosecutorial discretion, which is the idea that because of limited resources the executive cannot pursue every violation of federal law. The Justice Department must choose priorities and prosecute cases that are the most important, have the greatest impact, deter the most, and so on. But prosecutorial discretion is not being used in good faith here: A president cannot claim discretion honestly to say that he will not enforce an entire law - especially where, as here, the executive branch is enforcing the rest of immigration law.

Imagine the precedent this claim would create. President Romney could lower tax rates simply by saying he will not use enforcement resources to prosecute anyone who refuses to pay capital-gains tax. He could repeal Obamacare simply by refusing to fine or prosecute anyone who violates it.

The federal executive branch, like a local district attorney or a traffic cop, has some discretion about how, when, and whether to prosecute certain violations of the law. Not every driver exceeding the speed limit by 4 mph will receive a ticket, and not every teenager caught shoplifting will face a criminal indictment. Police and prosecutors are granted some leeway in these matters - but they cannot change the speed limit or legalize theft. That requires an act of the legislature. By making the liberal use of this discretion mandatory, the Obama administration is in effect writing new law rather than enforcing existing law.