FILE - This April 30, 2014, file photo shows Diana Jimenez, 2, as she sleeps in a stroller during a rally sponsored by local immigrant rights organizations, in Homestead, Fla. Jimenez's father was deported to Guatemala two years ago leaving Jimenez and four siblings in the U.S. (AP/AP)

Not since Abraham Lincoln pondered his Emancipation Proclamation in 1862 has a president considered ordering a more sweeping adjustment to membership in the American community than the mass relief for illegal immigrants that President Obama is said to be contemplating.

There is obviously no analogy between slavery and the disadvantages the undocumented face today. Among many other differences, the undocumented arrived voluntarily, searching, often successfully, for a better life. Also, they established residence unlawfully, for which there must be some reckoning.

Still, the essentially permanent presence of some 11 million people whose labor and cultural contributions benefit the United States — yet who must exist on the margins of society — creates dilemmas both practical and moral.

And the issue has this in common with slavery: It’s a long-standing debate over fundamental rights that the nation’s democratic institutions have proven incapable of resolving, leading to increasingly bitter partisan conflict.

The ideological concept behind Obama’s grand slicing of the Gordian knot of immigration, if he attempts it, would be akin to that which drove Lincoln’s action: the president as liberator.

The proposal that’s being bandied about — offering up to 5 million undocumented parents of U.S. citizens and others the same two-year renewable reprieves from deportation plus work permits that Obama already ordered for undocumented residents who arrived as children — wouldn’t cover all illegal immigrants any more than Lincoln’s proclamation freed every slave. (It omitted Union-held territory.)

Still, its impact would be dramatic, and might define Obama’s legacy as powerfully as the Emancipation Proclamation defined Lincoln’s.

There’s just one problem: Our system does not let the president make laws on his own, no matter how good his intentions. Lincoln himself was aware that he needed constitutional authority for the proclamation, invoking, plausibly, his power as commander in chief: Depriving the South of forced labor and making erstwhile slaves potential Union soldiers would help win the war, he argued.

The proclamation rather deftly calls itself “an act of justice warranted by the Constitution upon military necessity.”

With no war or other crisis to invoke, Obama would have to rely on more tenuous sources of authority, presumably an extension of the prosecutorial discretion he cited in his order relating to childhood arrivals. However, naturalization of aliens is an area over which the Constitution expressly grants authority to Congress, and about which Congress has legislated, abundantly, in the past.

Obama might not be in Supreme Court Justice Robert H. Jackson’s permissive “zone of twilight,” where Congress retains concurrent authority over a certain issue generally but has neither granted nor denied authority for a specific presidential action.

He could instead be taking measures “incompatible with the expressed or implied will of Congress” — albeit past Congresses — where, according to Jackson, “the authority of the President is at its lowest.”

The president himself put the point rather well in 2011 when he said that “there are enough laws on the books by Congress that are very clear in terms of how we have to enforce our immigration system that for me to simply through executive order ignore those congressional mandates would not conform with my appropriate role as president.”

Since then, Obama, like Lincoln before him, has been heavily lobbied by the most ideologically committed members of his political base. Like Lincoln in 1862, he is also facing losses in a mid-term election and weighing the benefits of pleasing party activists against the costs of alienating those more comfortable with the status quo.

Lincoln labored over his constitutional case for the proclamation not only because he felt it was his duty to do so, but also because he knew that a sound constitutional argument would fortify the measure politically.

He also wanted to forestall post-war court challenges by aggrieved slaveholders. The 13th Amendment abolishing slavery made that a moot point — which is one reason Lincoln fought so hard for its passage.

In that regard, Obama’s prospective order might have an advantage over Lincoln’s: Even if its constitutional basis is weak, it’s hard to see what anyone could do about it.

Congress could pass a law overriding his action, which he would veto. Impeachment also would fail, and wouldn’t necessarily undo his order even if it succeeded. The next president could rescind it — at great political hazard.

As for a lawsuit, who, exactly, would Obama’s order harm, in the sense of a “concrete, particularized injury” that the law requires for standing to sue in federal court?

Perhaps a state, or a business, that had to pay benefits to a newly waived-in immigrant could sue on the grounds that the immigrant is still illegal because Obama’s action was unconstitutional.

But we’re really getting hypothetical now; the slaveholder complaint that Lincoln feared was more readily imaginable.

In short, the broadest measure Obama is considering would be constitutionally dubious, politically explosive and flatly contradictory to his own recently expressed views.

It would also bring many hard-working people out of the shadows, make generations of Latinos into Democrats and secure a place in history for its author.

And it might be impossible to stop.

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