President Barack Obama. (Evan Vucci/AP)

DEMOCRATS URGING President Obama to “go big” in his executive order on immigration might pause to consider the following scenario:

It is 2017. Newly elected President Ted Cruz (R) insists he has won a mandate to repeal Obamacare. The Senate, narrowly back in Democratic hands, disagrees. Mr. Cruz instructs the Internal Revenue Service not to collect a fine from anyone who opts out of the individual mandate to buy health insurance, thereby neutering a key element of the program. It is a matter of prosecutorial discretion, Mr. Cruz explains; tax cheats are defrauding the government of billions, and he wants the IRS to concentrate on them. Of course, he is willing to modify his order as soon as Congress agrees to fix what he considers a “broken” health system.

That is not a perfect analogy to Mr. Obama’s proposed action on immigration. But it captures the unilateral spirit that Mr. Obama seems to have embraced since Republicans swept to victory in the midterm elections. He is vowing to go it alone on immigration. On Iran, he is reportedly designing an agreement that he need not bring to Congress. He already has gone that route on climate change with China.

The legal or constitutional case for each is different, but the rationales overlap: Congress is broken, so Mr. Obama must act. Two-thirds of Americans did not vote in the midterms, and the president must represent them, too. He has tried compromise, and the Republicans spurned him.

We will not relitigate that last contention except to note that behind the legislative disappointments of the past six years lies fault on both sides. The bigger point is this: In an era of fierce partisanship and close division, there will always be a temptation to postpone legislating until after the next election and to spend the intervening two years jockeying for political advantage. But a knockout blow will remain out of reach for both sides, and the price of postponement will be national decline. Many areas need federal attention and hold a possibility of bipartisan accord: building the nation’s infrastructure, protecting its cybernetworks and reforming its tax code, to name just three. It would not be rational for Republicans to spurn compromise in these areas just because Mr. Obama acts unilaterally in others; but it is entirely foreseeable.

We favor immigration reform, including a path to legal status for the 11 million foreigners in the country without valid papers. Congress should have acted on this long ago. We also understand that the president has broad authority in this area, which he exercised in 2012 to pardon young people who had been brought here as children.

Now, however, he is contemplating executive action not really aimed at one group or another but intended “to make the system work better,” as he said in his post-election news conference. He acknowledges that Congress should and could do this job, but he is tired of waiting.

Three years ago, when advocacy groups pressed him to take such a step, Mr. Obama demurred. “Believe me, the idea of doing things on my own is very tempting,” he said. “Not just on immigration reform. But that’s not how — that’s not how our system works. That’s not how our democracy functions. That’s not how our Constitution is written.”

Mr. Obama may find a constitutional way to rewrite the nation’s immigration laws. But in his frustration with democracy, he is likely to prove his point: Unilateralism will not make the system work.