President Trump said this week that he intends to issue an executive order that would end birthright citizenship for children born in the United States to undocumented immigrants. Such a move would upset a century of Supreme Court precedent and significantly restructure U.S. immigration and constitutional law.

That’s because Section 1 of the 14th Amendment declares, “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.” The Supreme Court and mainstream legal scholars on the left and right have long held that the phrase “under the jurisdiction thereof” extends citizenship to anyone born under U.S. territorial jurisdiction, including the children of immigrants.

But some on the far right have recently claimed that undocumented immigrants are under the jurisdiction of foreign countries, rather than the United States, precluding their children from birthright U.S. citizenship. Trump asserted this while campaigning in August 2015 and reiterated the idea this week.

But can a president change the rules of citizenship by fiat? Not so fast.

Here are three reasons Trump’s proposal might be unconstitutional.

1. The Constitution grants the power to regulate citizenship to Congress, not the president.

The 14th Amendment’s Section 5 gives enforcement of birthright citizenship exclusively to Congress. Similarly, Article I, Section 8, Clause 4 of the Constitution grants only Congress the authority to establish rules for citizenship by naturalization. Legal scholars Peter Schuck and Rogers Smith have, therefore, argued that Congress could pass a statute narrowing, clarifying or even enlarging the grounds for birthright citizenship.

Sen. Lindsey O. Graham (R-S.C.) announced via tweets that he intends to introduce a bill along the lines of Trump’s proposal, but no past efforts at comprehensive immigration reform have yet come close to passage.

What’s more, most legal scholars believe the 14th Amendment’s first section automatically grants birthright citizenship to all persons born on U.S. soil, meaning that any change would probably require a constitutional amendment. Even some Republicans in Congress, including outgoing House Speaker Paul D. Ryan (R-Wis.), seem wary of supporting Trump’s unprecedented grab for executive power.

2. The proposed executive order probably violates the intent of the framers of the 14th Amendment.

The infamous Dred Scott decision of 1857 held that black Americans could not hold birthright citizenship, effectively making citizenship a hereditary racial matter. By linking citizenship status to parentage rather than birthplace, Trump’s proposed executive order relies on similar legal reasoning. But in 1868, the framers of the 14th Amendment overruled the Dred Scott decision and outlawed such measures by granting automatic citizenship to anyone born under U.S. jurisdiction.

Congressional debates surrounding the 14th Amendment clarify this point. Some pundits have cherry-picked and sometimes falsified statements from the congressional record to claim that Congress did not intend to grant birthright citizenship to immigrants’ children. However, the more common and accurate position, elaborated on by then-Sen. Edgar Cowan (R-Pa.), held that any immigrant fell under “the protection of the laws” and police and courts of the United States, and, in this sense, was “subject to the jurisdiction” of the United States. Children of these immigrants were, therefore, entitled to birthright citizenship, as are the children of undocumented immigrants born on U.S. soil today.

3. Supreme Court precedent invalidates any such executive order.

In the 1898 United States v. Wong Kim Ark decision, the Supreme Court clarified that the 14th Amendment granted birthright citizenship to the children of immigrants, even when those immigrants were not themselves eligible for citizenship. Conservatives have claimed that this decision does not protect the children of undocumented immigrant parents who are subjects of other nations. But this is a misreading of the case. As the Wong Kim Ark ruling held:

To hold that the Fourteenth Amendment of the Constitution excludes from citizenship the children, born in the United States, of citizens or subjects of other countries would be to deny citizenship to thousands of persons of English, Scotch, Irish, German, or other European parentage who have always been considered and treated as citizens of the United States.

Here, the court suggested the 14th Amendment granted birthright citizenship even to the children of parents who were subjects of other nations. Wong Kim Ark, therefore, establishes a clear, long-standing precedent for birthright citizenship for children of undocumented parents.

Trump might be trying to rally his base for the midterms.

Then again, Trump’s talk of an executive order one week before contested midterm elections might simply be a partisan ploy. Trump has asserted, incorrectly, that birthright citizenship draws people to illegally enter the United States. He may, therefore, be proposing the executive order as a nativist stunt to rouse the flagging Republican base days before the elections. Indeed, on Tuesday, Trump seemed unsure how the order would overcome these high constitutional hurdles, while also incorrectly stating that only the United States grants birthright citizenship. In fact, about 30 other countries do so.

Still, the Supreme Court upheld a watered-down version of the president’s travel ban, initiated by executive order. That ruling gave Trump wide leeway to interpret and enforce the nation’s immigration laws. There is a chance that the court — now with five reliably conservative justices — could affirm some or all of a Trump executive order that reinterprets birthright citizenship law. Such a move would narrow the 14th Amendment’s scope, perhaps reversing Wong Kim Ark. The courts, not Trump, are likely to have the final word.

Robinson Woodward-Burns (@rwbdc) is an assistant professor of political science at Howard University, where he researches American constitutional thought and history, federalism, abolitionism and American transcendentalism.