BY DECLARING that “self-deportation” is the solution to illegal immigration, Mitt Romney gave voice to an idea in wide currency among Republicans — that America’s 11 million undocumented immigrants would simply go home if government made their lives miserable enough. But even by the debased standards of primary-season rhetoric, the idea is as simple-minded and absurd as it is popular — as Mr. Romney’s rival, Newt Gingrich, quickly pointed out.

“Self-deportation” is snappy and sound-biteable; hence its superficial appeal. Slap together a water-tight employment verification law, issue IDs to legal workers, add some harassment from state and local authorities, and watch the unpapered immigrants stream south over the border whence they came. If they want to return to the United States, said Mr. Romney, they can get to the “back of the line.”

The idea’s inanity is masked by its allure for some who hate illegal immigration but concede that mass roundups and deportations would be unseemly and prohibitively expensive. Better, they say, that illegal immigrants leave under their own steam — and pay their travel expenses, too.

The problems start with economics. Undocumented workers comprise more than 5 percent of America’s labor force and much more in agriculture, hospitality, landscaping and elsewhere. Their departure would be a blow to the economy — and no, American-born workers, generally better educated and geographically remote from the jobs immigrants do, would not fill their shoes.

Mr. Romney also wrongly imagines that illegal immigrants, who are mostly Mexicans, can be easily uprooted. In fact, nearly two-thirds have been here for more than a decade, and more than 28 percent arrived more than 15 years ago. Their roots here are deep: Almost half have children, and more than 80 percent of those 5.5 million children were born here and are U.S. citizens. Does Mr. Romney expect them to “self-deport” with their parents?

As for the idea of getting to “the back of the line,” that assumes a line exists for low-skilled Mexicans lacking family connections in the United States. There is no such line that would realistically result in the issuance of a green card to an unskilled Mexican with no relatives in the United States. Even if a Mexican has, say, parents or siblings who are U.S. citizens, he would likely wait 15 to 20 years or more just to get an interview with a U.S. consular officer.

Undocumented immigrants in this country are highly resilient. During the economic meltdown of 2008-09, the shrinking job market and beefed-up border security helped cut the illegal population by about a million. But since then, the numbers have leveled off — despite a still-sluggish economy, stepped-up deportations, and hostile state and local legislation. The vast majority of illegal immigrants are here to stay.

More broadly, the problem is a broken system and politicians who cannot bring themselves to reconcile the hard reality of illegal immigration with their own slogans. Sensible reform would include tight enforcement as well as adequate admissions of guest workers for industries that need them and, critically, a path to legalization for undocumented immigrants. They’re not leaving, notwithstanding the fantasy of “self-deportation.”