Under Article II, Section 3 of the Constitution, the president has the duty to “take Care that the Laws be faithfully executed.” This provision was included to make sure that the president could not simply choose, as the British King had, to cancel legislation simply because he disagreed with it. President Obama cannot refuse to carry out a congressional statute simply because he thinks it advances the wrong policy. To do so violates the very core of his constitutional duties. …

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.