Ronald D. Rotunda is a professor of jurisprudence at Chapman University and co-author of a six-volume treatise on constitutional law. He was assistant majority counsel to the Senate Watergate Committee from April 1973 to July 1974.

Old-time monarchies and modern dictators govern by decree. Not so in the United States, where the Constitution does not give the president such power. So it is fair to wonder what authorized President Obama to “suspend” the portion of the Affordable Care Act that imposes a health-care mandate on employers.

There are times when the president can issue decrees that have the force of law. For example, Congress may legislate that certain things will or will not happen unless the president issues certain findings. In 1936, the Supreme Court upheld a law passed in 1934 that made it a crime to sell munitions to Bolivia, which was engaged in an armed conflict, if the president made certain findings.

The Constitution also gives the president specific unilateral powers, such as the power to decide which foreign countries to recognize and the authority to grant pardons. The president can refuse to prosecute someone criminally because the Constitution gives the executive branch absolute prosecutorial discretion. That is how Obama justified not enforcing deportation laws in certain circumstances.

The president also need not enforce a law that he believes is unconstitutional. Sound precedent justifies this power. The Justice Department’s Office of Legal Counsel (OLC) is, in effect, the lawyer for the government. It issues legal opinions that courts often rely on. It advises that the president may refuse to enforce a law he thinks is unconstitutional. It derived this power from two clauses of the Constitution: One requires the president to “take Care that the Laws be faithfully executed.” The other requires him to “preserve, protect and defend the Constitution of the United States.” The OLC agreed with Chief Justice Salmon P. Chase, who wrote in 1868 that the president’s obligation to defend the Constitution authorizes him to decline to enforce statutes that he believes violate it.

The OLC has also concluded that the president could refuse to enforce a law before the judiciary decides whether it is constitutional. Thus, before the Supreme Court ruled on the issue last month, Obama acted properly in legally ordering the Justice Department not to defend a federal law he considered unconstitutional because it did not recognize same-sex marriage.

The president does not, however, have carte blanche to refuse to enforce the law. “Obviously,” the OLC wrote in 1990, the president cannot “refuse to enforce a statute he opposes for policy reasons.” In 1838, the Supreme Court advised: “To contend that the obligation imposed on the President to see the laws faithfully executed implies a power to forbid their execution is a novel construction of the constitution and entirely inadmissible.” When President Richard Nixon refused to spend funds that Congress had ordered him to spend, the court held without dissent in 1975 that the president must follow the federal statute, not his policy preferences.

Obama’s decision to “suspend” the employer mandate of the Affordable Care Act has no support in precedent and dramatically shifts the arc of presidential power. The Affordable Care Act has no provision giving the president power to suspend or postpone the mandate. The law requires employers with 50 or more full-time workers to provide health-insurance coverage or pay a penalty. Section 1513(d) of the act — titled “Effective Date” — stipulates that this mandate “shall apply” after “December 31, 2013.”

Which part of “shall” gives the president the power to ignore the law that he signed with such fanfare? If the president can change the effective date to Dec. 31, 2014, what prevents him from changing the date again, along with other provisions? As Sen. Tom Harkin (D-Iowa) asked: “This was the law. How can they change the law?

If the president has the discretion to ignore laws that he prefers not to exist, the constitutional limits of presidential authority have the restraining power of air. President George H.W. Bush argued, unsuccessfully, that Congress should lower the capital gains rate. Under Obama’s faulty interpretation of presidential power, he should have simply instructed the Internal Revenue Service not to tax capital gains at a rate greater than 10 percent, regardless of the tax code. The Supreme Court held in 1998 that it was unconstitutional to give the president a line-item veto that Congress could override. Obama’s newfound “power” is much greater than that, because there is no procedure to override a presidential decree.

Going forward, why should lobbyists urge Congress to create, extend or change a tax deduction when all they have to do is persuade the president? Obama, a proven fundraiser, could outdo himself over the next several years with this new “power.” Nixon might have salivated at such power — which simply isn’t part of the president’s authority.