In terms of the impact outside of immigration — can a Republican president unilaterally decline to enforce the penalties for failing to buy health insurance? Exercise prosecutorial discretion not to penalize failure to pay the “death tax”? Stop going after violators of environmental laws?

Here, too, the administration pooh-poohs any spillover effect. The president would not be declining to enforce an entire law or category of law (as in my hypothetical of no Clean Air Act prosecutions, or tax enforcement of the individual mandate). He would simply be prioritizing how to use limited resources in continuing to enforce the law.

I see the distinction but remain uneasy — and I’m not the only one. University of Virginia law professor David Martin is a Democrat and a supporter of comprehensive immigration reform who served as principal deputy general counsel of the Department of Homeland Security during the Obama administration’s first two years.

“For Democrats, it’s a dangerous precedent,” he told me. “You’re opening the possibility for a Republican president to say, I’m not going to go forward with enforcement in a number of areas.”

There are compelling humanitarian reasons for Obama to act. But the president and his allies must keep in mind: Presidential power, once expanded, is hard to contain.

OBAMA: First of all, temporary protective status historically has been used for special circumstances where you have immigrants to this country who are fleeing persecution in their countries. Or there’s some emergency situation in their native land that required them to come to the United States. So it would not be appropriate to use that just for a particular group that came here primarily, for example, because they were looking for economic opportunity.

With respect to the notion that I can just suspend deportations through executive order, that’s just not the case. Because there are laws on the books that Congress has passed. And I know that everybody here at Bell is studying hard so you know that we’ve got three branches of government. Congress passes the law. The executive branch’s job is to enforce and implement those laws. And then the judiciary has to interpret the laws.

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, to ignore those congressional mandates would not conform with my appropriate role as president. That does not mean, though, that we can’t make decisions, for example, to emphasize enforcement on those who’ve engaged in criminal activity. It also doesn’t mean that we can’t strongly advocate and propose legislation that would change the law in order to make it more fair, more just, and ultimately would help young people who are here trying to do the right thing and whose talents we want to embrace in order to succeed as a country.

Obama said that the protesters were right about a lot of illegal immigrants getting deported but that he was acting to change it.

“What you’re not paying attention to is the fact that I just took an action to change the law,” Obama said.