A Slippery Slope On Immigration

Given this context and history, the administration argues that I can relax on both fronts. On immigration policy, however broad the scope of the president's action -- extend the protections granted to the so-called "dreamers" to their parents; grant the ability to remain in the country to the parents and children of U.S. citizens -- it's clear, this argument goes, that the president will not suspend all deportations and will continue to fully expend the congressional funds appropriated for this purpose. This is not a situation of Richard Nixon simply announcing that he would "impound" funds duly appropriated by Congress because he did not agree with its funding priorities.

On this front, size matters. The closer the president's action edges to protecting all of those in the country illegally, the more legally dubious it becomes.

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.

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Ruth Marcus' email address is ruthmarcus@washpost.com.