For anyone who wonders why the president doesn’t just take Rich Trumka’s advice and force Congress into action on immigration reform by halting deportations, Wonkblog’s Brad Plumer has a very comprehensive answer today.

Obama doesn’t have the power to halt deportations. He does have the power of deferring some deportations, as a matter of setting priorities at a time when federal agencies do not have the resources to deport all undocumented people even if they wanted to. That’s how the administration was able to launch the DACA (Deferred Action for Childhood Arrivals) initiative in 2012 which promised “DREAMers” temporary immunity from deportation (550,000 people have applied for DACA status, and 72% have been approved, says Plumer).

Expanding DACA is theoretically possible, but extending it to anything like the entire undocumented population, politics aside, is very dubious legally.

That means — and again, this is all just in theory — Obama could extend promises of deferred deportation to some additional groups of illegal immigrants. He could try to extend it to domestic-violence victims, say. Or to undocumented family members of legal immigrants. Or to workers who are bringing civil rights or labor-violation claims. Or to those with disabled children. It’s possible that these moves could cover another couple million undocumented immigrants. But he can’t extend deferral to everyone.

“There’s certainly room for adjustment, but not anything sweeping,” said David A. Martin, a law professor at the University of Virginia and the principal deputy general counsel of the Department of Homeland Security in 2009 and 2010. “The justifications for DACA made clear that this is not a situation where the president can reduce overall enforcement of immigration laws. He can just redirect it in certain ways.”

If the Obama of conservative demonology were real, the president, of course, could just disregard the law, and maybe give every undocumented American an Obamaphone and some food stamps. But that’s not how he rolls.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.