Tonight, President Obama will override Congress and effectively declare amnesty for at least several million illegal immigrants. As Scott has noted, Obama himself has repeatedly admitted he lacks the constitutional power to make this move.

Some of Obama’s defenders claim that the president was mistaken when he acknowledged his lack of power to override the immigration laws. One supporter (I don’t remember who) said that Obama received bad legal advice. Apparently, the “constitutional lawyer” was misled by his legal team.

The most common argument in favor of Obama’s power to declare amnesty by executive order is that Ronald Reagan and George H.W. Bush did it. Juan Williams, among others, has made this claim.

It is baseless. Unlike what Obama is about to do, Presidents Reagan and Bush 41 issued immigration regulations that were expressly authorized by a law passed by Congress.

In 1986, Reagan signed into law the Immigration Reform and Control Act. The Act required him to adjust the status of certain illegal immigrants to the category of “alien lawfully admitted for temporary residence.”

The Act also authorized the Attorney General to allow other illegal immigrants who did not qualify for the amnesty to remain in the U.S. if needed “to assure family unity.”

Accordingly, in May 1987, the Justice Department issued regulations that interpreted the the term “family unity” as calling for the maintenance of the “family group.” Family group was defined as including “the spouse, unmarried minor children under 18 years of age who are not member of some other household, and parents who resided regularly in the household of the family group.” Thus, not all spouses and children were included.

This regulation was not an exercise of prosecutorial discretion or the assertion of a generalized right to suspend “oppressive” immigration laws. Rather, the administration made it clear that it was carrying out the direction of Congress. It even cited the section of the law that provided this direction (section 245(d)(2)(B)(i) of the 1986 Act).

House Democrats, including one of the authors of the 1986 Act, criticized Reagan for interpreting too narrowly the executive authority they had granted him. They wanted all spouses and children to receive amnesty in the name of family unity. If anything, then Reagan acted too cautiously, exercising less than the full discretion afforded him by Congress.

Enter President George H.W. Bush. In 1990, he expanded the Reagan DOJ’s interpretation of “family unity” to encompass all spouses and children. Like Reagan, Bush merely interpreted the 1986 Act, as Congress called on the executive to do.

Bush’s interpretation was the more expansive one that Democrats had urged the Reagan administration to adopt. Both the Reagan and Bush interpretations were reasonable, though differing, attempts to effectuate congressional intent.

Obama, by contrast, will not be fulfilling a congresional mandate to interpret a new statute. He will be overriding the immigration law in the name of “prosecutorial discretion” on the ground that Congress hasn’t enacted a new statute.

It will be an unprecedented, unlawful, and dangerous assertion of executive power. It might well create a constitutional crisis.