To think that President Barack Obama has taken the oath of office four times (through accidents of circumstance, twice each time he was elected). Taking the oath must have become such old hat that he stopped paying attention.

The president is issuing an executive amnesty for illegal immigrants based on blatant contempt for the constitutional order that he is sworn to uphold. Where does Abraham Lincoln go to get his Bible back?

The past 400 years of Anglo-American political history can be read as a successful effort to establish and maintain a system tethering the executive to the law. What President Obama is doing will undermine that achievement, both through his own lawlessness and the precedent he will create for subsequent presidents to operate by extralegal fiat.

There are many opponents of the president’s unilateral action, but few as eloquent as the president himself through the years. It doesn’t take someone who used to teach constitutional law to know that Congress writes the law and the president executes it, even if he finds it personally distasteful.

In one of his many disavowals of having the power he is now wielding, the president said at a Univision town-hall meeting in 2011 that “Congress passes the law. The executive branch’s job is to enforce and implement those laws. And then the judiciary has to interpret the laws.”

Thank you for the civics lesson, Mr. President. His new theory is that the president huffily demands that laws pass, and if Congress refuses, he can create a new legal dispensation to his liking.

The president and his supporters pretend that the Immigration and Nationality Act contains a gigantic asterisk that says, notwithstanding the elaborate legal infrastructure set out in the law and the distinctions among different categories of immigrants, the president can do whatever he wants. No Congress would ever write the law this way.

The president’s defenders rely on the notion of prosecutorial discretion, the existence of which is uncontroversial. The executive doesn’t have the resources to hunt down and prosecute every violator of our laws, and therefore has to establish enforcement priorities.

The Congressional Research Service did a report on prosecutorial discretion and immigration that, for the most part, emphasizes its piddling reach. It says, for instance, that immigration officers may use discretion to decide whom to stop, question and arrest, or whether to issue or cancel a Notice to Appear.

No one heretofore has thought this leeway could be used to eviscerate an entire statutory scheme. Under the Obama precedent, future presidents can use the pretense of prosecutorial discretion to dispense with swaths of the federal code and come up with alternatives. Can’t prosecute all pot dealers? Ignore the drug laws. Can’t find every tax scofflaw in the country? Rewrite the tax code.

Other presidents have, in keeping with the law, provided temporary relief to foreign nationals whose native countries have been torn by civil strife or natural disasters. In 1990, George H.W. Bush gave safe harbor to Chinese students after Tiananmen Square. Bill Clinton did the same for Central Americans here after hurricanes hit the region in 1998. The numbers were typically in the hundreds or thousands.

All this makes for a sound basis in precedent and the law for President Obama’s decision to give Syrians safe harbor in 2012. It doesn’t come close to justifying his executive amnesty.

The gotcha example of George H.W. Bush granting amnesty to some spouses and children of recently legalized immigrants in 1990 isn’t apt either, since the scale was much smaller (only about 140,000 people took advantage of it), and Congress voted to codify it within months.

No matter how much the president’s defenders stretch for a legal justification and a precedent, the conclusion is unavoidable that no one has done this before. President Obama is said to want to build his legacy, and he is — as a man who is shamefully careless of his oaths and constitutional obligations.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.