WASHINGTON—The immigration debate that will have lasting consequences is not the one between the left and the right but the one taking place within the ranks of conservatives in the United States and around the world.

A few days ago, during a visit to a school in Silver Spring, Md., Michelle Obama was confronted by a 7-year-old girl who asked her why the president “is taking everybody away that doesn’t have papers.” With Mexican First Lady Margarita Zavala next to her, Obama responded, “That’s something we have to work on, right?” The girl produced a devastating rejoinder: “But my mom doesn’t have any papers.”

The anti-immigration right, led by radio personality Rush Limbaugh, accused the White House of staging the exchange, claiming the girl had botched the delivery because she was supposed to say “Arizona” rather than “Obama.” Limbaugh was implying that it was a stunt to obtain sympathy against that state’s recent anti-immigration law. The pro-immigration camp in the Republican Party, meanwhile, sidestepped questions about the incident, avoiding confrontation with the nativists but maintaining enough distance to signal a more moderate position.

This anecdote illustrates the tension within the right at large. Sen. John McCain, who proposed giving millions of undocumented foreigners a path to legalization during the George W. Bush administration, is scared of losing a primary against an anti-immigration candidate in Arizona, where the recent law is popular among conservatives. McCain’s lukewarm support for a law he intimately dislikes is very telling about the relative strength of the anti-immigration forces within the party.

The chasm within the right goes back a century. The restrictive laws of the 1920s, a watershed in immigration history, saw a split between nativist Republicans and Southern Democrats, on the one hand, and the business community, which wanted immigrant labor, on the other. The intellectual right, including Walter Lippmann, was largely pro-immigration.

When Ronald Reagan, a pro-immigration conservative, gave amnesty to 3 million immigrants in 1986, he had to include in the law sanctions against employers. This reflected the pressure from the anti-immigration right. In the 1990s, Proposition 187, a California ballot initiative that would have denied public services to undocumented immigrants, became a flashpoint of the conservative-against-conservative fight. Its most prominent backer was Republican Gov. Pete Wilson, but several GOP heavyweights opposed it. Jack Kemp and William Bennett put out a statement saying, “The vast majority of immigrants hold principles which the Republican Party warmly embraces: an entrepreneurial spirit and self-reliance, hostility to government intervention, strong family values and deeply-rooted religious faith.”

In recent years, the debate pitted Bush, McCain and other pro-immigrant Republicans against the likes of Rep. Tom Tancredo of Colorado. Today, the debate rages on among conservatives. The Wall Street Journal is a staunch supporter of immigration, while Sean Hannity, a Fox News talk show host, is a ferocious critic.

This is not just an American debate. In Spain, conservatives are equally divided. The regional government of Madrid headed by Popular Party notable Esperanza Aguirre, with help from her immigration secretary, Javier Fernandez-Lasquetty, and adviser Mauricio Rojas, a former Swedish parliamentarian of Chilean origin, has tried to push the party toward embracing immigration. There is strong resistance from those who vilify the decision by the Spanish government to legalize 700,000 immigrants in 2005.

In Britain’s Conservative Party, a gulf separates pro-immigration figures such as former Chancellor of the Exchequer Kenneth Clarke and Oxford University Chancellor and former Hong Kong Gov. Chris Patten, from anti-immigrant spokesmen such as former diplomat Andrew Green, chairman of MigrationWatchUK.

I am not implying there is no debate among the left. The division among liberals in the United States is old too. The unions opposed immigration for decades. They were instrumental in ending the Bracero Program in the 1960s, as was liberal TV journalist Edward Murrow, who devoted a sensational report, “Harvest of Shame,” to denigrating the work-permit program. Many on the left, furthermore, support immigration for reasons related to collectivist “multiculturalism” rather than to the free movement of people.

But today, if the division were less profound on the right, immigration reform would pass. It is the type of issue that requires a broad consensus. Yet the paramount factor today is that a faction of the right has forcefully held its ground against anything that smacks of legalizing the people who live in the shadows and creating an avenue for significant future inflows. Until the debate among conservatives is settled, no meaningful reform is possible.