One of the most common methods Obama has used to make things go his way are through Executive Orders. It's also one of the scariest things that Trump is going to have at hand when he's sworn in. But where did the President get that power anyway?
The origins of modern Executive Orders lie in the War Powers Act of 1917 during WWI. That enabling legislation gave the president the *temporary* "emergency" powers to immediately enact laws regulating trade, economy, and other aspects of policy as they pertained to enemies of America. But they explicitly *excluded* that power from being able to affect Americans directly, but those powers were never rescinded after the end of the war.
Then, under FDR in 1933, the War Powers act was amended to allow those same, essentially legislative making power, to operate directly on Americans.
That is the legislation behind the fact that Executive Orders (and later judged to include Memoranda and Directives) are treated as law. It has nothing to do with the Constitution, as the Constitution says "*ALL* legislative Powers herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives."
The power granted to the Executive branch was only to carry out that legislation, not make it, as well as deal with foreign affairs (although without Senate consent, no treaties are valid) and as Commander in Chief of the military, to grant Reprieves and Pardons, and to make appointments to the courts, ambassadors, ministers, consuls, etc. when consented to by the Senate.
As executive types of power are the most common type of overweening authoritarian actions, it was intentionally made very weak and dependant on the Legislative and overridable by the Judicial branches. It was NEVER intended to be authorized to act on its own domestically without explicit legislative authorization.
It's one thing to lay out how a specific piece of legislation is to be enforced, but it's an entirely different thing to *create* regulation with the force of law out of whole cloth.
Past infractions do not justify present or future ones. Although forgiven in retrospect as it having been done for a "good" cause, much of what Lincoln did during the Civil War was outright unconstitutional. For that matter, so was the Louisiana Purchase, as it was presented for payment as a fait accompli, and several other significant actions taken by presidents essentially on their own.