Republicans are divided on birthright citizenship, one of their party’s greatest achievements.

Birthright citizenship has split the GOP presidential field. Following Donald Trump’s call for an end to birthright citizenship for the U.S.-born children of undocumented immigrants, fellow Republican presidential hopefuls Bobby Jindal, Rand Paul, Rick Santorum, Ben Carson and even longtime immigration reform advocate Lindsey Graham have said they support ending the practice. Marco Rubio and Jeb Bush have been the most vocally opposed. 

This didn’t used to be such a difficult issue for Republicans. After all, it was the GOP that wrote birthright citizenship into America’s constitution. The leaders of the 1866 Republican Party—the Party of Lincoln—were staunch supporters of the idea. Indeed, birthright citizenship was central to the Republican vision for post-Civil War America, and a key dividing line between the supporters of President Andrew Johnson and those of the Republican leadership in Congress.

Birthright citizenship had long been the traditional rule in the United States—one rooted in the English common law and adopted by many colonies and early states. Citizenship was acquired by soil rather than bloodline—subject to a few well-established exceptions, such as for the children of foreign diplomats or invading armies. But various Southern courts in antebellum America chose to diverge from this tradition in certain cases, allowing their states to deny birthright citizenship to those they deemed unworthy, such as African Americans.

This Southern “tradition”—fueled by white supremacy—was reinforced by the opinions of certain pro-slavery Attorneys General and ultimately codified in the Supreme Court’s infamous Dred Scott decision, authored by Chief Justice Roger Brooke Taney, himself a former pro-slavery Attorney General under President Andrew Jackson. In Dred Scott, Taney concluded that African Americans could not be U.S. citizens even if they were born free on American soil.

During Reconstruction, one of the Republican Party’s central goals was to overturn Dred Scott and guarantee equal citizenship for everyone born on American soil. President Lincoln signaled this move early in his administration through an 1862 opinion by his Attorney General, Edward Bates. Replying to a request by Treasury Secretary Salmon Chase, Bates defended birthright citizenship for African Americans, explaining, “You and I have no better title to the citizenship which we enjoy than ‘the accident of birth.’” After the Civil War, congressional Republicans followed Bates’ (and Lincoln’s) lead.

By late 1865, Lincoln’s promise of a “new birth of freedom” was very much in doubt. Following Lincoln’s assassination in April 1865, President Johnson had pardoned thousands of Confederate officials and plantation owners. More troubling, he deferred to the Southern states on how best to rebuild their societies, leaving them free to enact the Black Codes, which sharply limited the civil rights of the newly freed slaves.

With the ex-rebels gaining political strength and an important midterm election looming the following fall, congressional Republicans quickly settled on a potent one-two punch. First, they would pass a civil rights bill that would counter the Black Codes and secure important protections for the newly freed slaves. Second, they would push for a constitutional amendment that would establish constitutional baselines for post-Civil War America. At the center of both of these measures was a key principle—birthright citizenship.

Senator Jacob Howard, a radical Republican from Michigan, was at the center of this political fight. He helped to draft and pass the Thirteenth Amendment, ending slavery. He also served on the Joint Committee on Reconstruction and supported the Civil Rights Act of 1866. Most importantly, when Senator William Pitt Fessenden—Chair of the Joint Committee on Reconstruction—fell ill, Howard took over as the chief spokesperson for the Fourteenth Amendment, which was to establish the laws governing citizenship.

In this new role, Howard introduced the measure before a packed Senate gallery on May 23, 1866—a speech that was published on the front pages of various newspapers, including the New York Times and the New York Herald. While Howard explained quite well the “privileges or immunities” of U.S. citizenship that would be protected by the proposed amendment, the draft amendment did not yet define how one became a citizen in the first place. Was it by virtue of birth or blood? A product of national policy or state prerogative? And, what would happen if the Republican Party fell out of power and the supporters of the Old Confederacy took over the federal government?

A week later, Howard rose in the Senate and proposed an answer to these questions—the Citizenship Clause, enshrined in the Fourteenth Amendment today as follows: “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.” This provision echoed similar language in the Civil Rights Act of 1866, approved by Congress a mere two months earlier over President Johnson’s veto.

Through this new clause, Howard sought to overturn Dred Scott and guarantee equal citizenship for everyone born on U.S. soil. Howard conceded that there were small groups that would be excluded, consistent with well-established law, such as the children of foreign diplomats and certain Native American tribes. However, Howard was clear about the core purpose of the new Clause: “to put this question of citizenship and the rights of citizens and freedmen under the civil rights bill beyond the legislative power of . . . gentlemen . . . who would pull the whole system up by the roots and destroy it, and expose the freedmen again to the oppressions of their old masters.”

Fair enough. But what did this new provision mean for the U.S.-born children of resident immigrants—Trump’s main concern? Quite a bit. While the Citizenship Clause was paradigmatically about African Americans, the clause’s text and history confirm that it was about much more than that—namely, equal citizenship for everyone born on U.S. soil, regardless of race, color or parental origin.

Opponents of the Citizenship Clause expressed anxieties about the effects of the clause on the U.S.-born children of unpopular immigrant populations, such as the Chinese out west and the Gypsies in the east, with Senator Edgar Cowan—a conservative Republican from Pennsylvania—using especially unflattering rhetoric to describe the Gypsies as “people who invade [Pennsylvania’s] borders; . . . who pay no taxes; . . . and who do nothing, . . . but . . . settle as trespassers wherever they go.”

In the face of Cowan’s tough rhetoric, supporters of birthright citizenship stood their ground. For instance, Senator John Conness of California—a naturalized citizen from Ireland—replied that he supported citizenship for “the children . . . of Chinese parents” and the “children of all parentage whatever,” born in California. Leading Republicans such as Howard and Senator Lyman Trumbull—a moderate Republican from Illinois and sponsor of the Civil Rights Act of 1866—expressed similar sentiments during the debates over the Civil Rights Act and the Fourteenth Amendment. In the end, Congress approved the Fourteenth Amendment—including the Citizenship Clause—on June 13, 1866. And the Fourteenth Amendment was finally ratified by “We the People” on July 9, 1868.

The Fourteenth Amendment is one of the Republican Party’s greatest achievements. With it, Republicans continued to write Lincoln’s promise of “a new birth of freedom” into our Constitution and lead a Second Founding of our republic. Birthright citizenship was a key part of the Republican program. Before resolving to eliminate it through constitutional amendment (as some propose) or simply read it out of our Constitution (as Donald Trump suggests), it’s worth pausing for a moment to reflect on the important place that this concept has in our nation’s constitutional story.