AT A stroke, President Obama will protect, at least for the remainder of his term, more than 4 million illegal immigrants from the threat of deportation. He justifies the move as an act of “prosecutorial discretion.” The president always has had authority to calibrate and prioritize the enforcement of immigration (and other) laws, but this wholesale reinterpretation amounts to ­overreaching.

Mr. Obama, a former constitutional law professor, has said as much, explicitly and many times. “Now, I know some people want me to bypass Congress and change the laws on my own,” he said in 2011. “That’s not how our democracy functions. That’s not how our Constitution is written.” He was speaking to La Raza, one of the groups pressing for unilateral action. Now those groups have won the day, though the victory may be pyrrhic.

On Thursday evening, they organized watch parties to see Mr. Obama’s speech on the matter. On Friday, he is scheduled to travel to Nevada to celebrate in a purple state. The move has the feel of a political campaign, not a soberly considered act of governance.

Inevitably there is arbitrariness in the fine print. Why is it legal to protect the undocumented parents of citizens from deportation, as the executive order does, but not the parents of undocumented children, whom Mr. Obama has already shielded from immigration enforcement? Why is it all right to extend work permits to immigrants here illegally, but not health insurance under the Affordable Care Act? Immigration law, by its nature, involves the setting of arbitrary limits that leave millions unfairly on the wrong side; all the more reason to insist on the legitimacy of constitutional lawmaking. Mr. Obama was right to argue Thursday that otherwise law-abiding undocumented immigrants deserve a route out of the shadows. But unilateral action is not the right way to achieve that.

Republicans, obstinate and inert for so long on immigration, cannot dodge responsibility. Even after the Senate passed sweeping immigration reform last year with bipartisan support, House Speaker John A. Boehner (R-Ohio) refused to allow a vote on the bill in the lower chamber, where it would have been likely to pass. Republicans now berate the president for thwarting the popular will; yet the GOP thumbed its nose at democracy by refusing to submit the question to an up-or-down vote.

Now the White House seems almost eager to goad the opposition into a collective temper tantrum — and may succeed. Sen. John Cornyn (R-Tex.) has threatened a constitutional crisis, Sen. Tom Coburn (R-Okla.) is predicting violence and others are beating the drums for impeachment.

There is a smarter way, for the nation and the Republican Party. We realize it will not be the GOP’s first impulse. But by fixing the nation’s broken immigration system on their own terms, Republicans could negate the president’s fiat, which, after all, is provisional and partial; assert their prerogative as elected lawmakers; repair their standing with Hispanic voters; and demonstrate an ability to be constructive.

If Republicans want revenge, in other words, they have a ready way to take it. It’s called legislation.