Donald Trump’s call for doing away with birthright citizenship for the children of illegal immigrants has once again focused media attention on the idea and led some of his GOP rivals to signal openness to it. The Huffington Post looked at the positions of the 2016 GOP candidates and concluded that “a good chunk of the GOP field” wants to revisit birthright citizenship.

As conservative writer Ben Domenech notes, many other GOP lawmakers have supported the idea in recent years, as do Republican voters, even though birthright citizenship is “one of the things that is relatively unique to the American experiment,” and requires rebelling against the “Constitutional mandate placed within the Fourteenth Amendment in the wake of the Civil War.”

But there’s another historical irony here that was pointed out by renowned American historian Eric Foner: The 14th amendment and birthright citizenship rank among the great and defining accomplishments of the Republican Party, back when it was the Party of Lincoln.

“This was one of the historic achievements of the Republican Party,” Foner, who has written extensively on Reconstruction and the meaning of American freedom, tells me. “There’s plenty of irony here.”

Foner thinks that ending birthright citizenship would require changing the 14th amendment, which was passed and ratified after the Civil War to secure the citizenship of former slaves. Recall that the 14th amendment came after the Civil Rights Act of 1866, which was passed by Republicans in Congress and (among other things) defined all people born in the United States (except Native Americans who didn’t pay taxes) as citizens. That Act was passed over the veto of Democratic president Andrew Johnson, and the 14th amendment was then secured by Congressional Republicans — and ratified by the states — to make African American citizenship irreversible.

In the current context, of course, the debate over birthright citizenship concerns the proper status not of former slaves, but of the children of undocumented immigrants. But Foner notes that at the time, the Republican Party’s achievement of the 14th amendment showcased the party’s adherence to broad principles that do seem applicable to the current debate.

“The Civil War and the emancipation of the slaves shaped the Republican Party and pushed it in the direction of nationalism, inclusiveness, and openness,” Foner says. “The idea was that citizenship should be extended to people regardless of accidental characteristics, such as race, national origin, or the status of their parents. This established a national standard for citizenship. The principle was one of opportunity and inclusiveness. That’s what the Republican Party stood for. The 14th amendment became one of the defining principles of the Republican Party.”

At the time, the 14th amendment did also have an impact on immigration status, Foner adds. “The equivalent back then would have been the Chinese in the U.S.,” Foner says. “They could not become naturalized citizens. Nonetheless, their children born here were citizens under the 14th amendment. The point is, this isn’t just about black people. It’s a statement about what America is. It’s a place where anybody can become a citizen regardless of their parents, religion, or race.”

Even some Republicans have fretted aloud that the party has moved away from some of the principles of the Party of Lincoln in its current treatment of minorities in a broader sense. Rick Perry recently opined that the GOP has been less-than-committed to another key element of the 14th amendment — the federal government’s role in enforcing Civil Rights: “when we gave up trying to win the support of African-Americans, we lost our moral legitimacy as the party of Lincoln.”

Democrats, too, have been wrestling with their racial heritage. Some state parties have changed the name of their “Jefferson-Jackson Dinners,” because Thomas Jefferson and Andrew Jackson were slave-owners.

Foner locates another historical irony in the current push among some Republicans to do away with birthright citizenship — one involving “American exceptionalism,” which is often invoked by Republicans, some of whom like to claim that the current president is not a sincere believer in it.

“This is one of the real examples of American exceptionalism,” Foner says. “There are very few countries that have birthright citizenship today. This does make us different.”