President Donald Trump is making another hardline immigration play in the final days before the midterm elections. Trump declared that he wants to order an end to the constitutional right to citizenship for babies born in the United States to non-citizens. Most scholars think he can’t implement such a change unilaterally.

With seven days to go before high-stakes elections that he has sought to focus on fearmongering over immigration, Trump made the comments to “Axios on HBO.” Trump, seeking to energize his supporters and help Republicans keep control of Congress, has stoked anxiety about a caravan of Central American migrants making its way to the U.S.-Mexico border.

His administration announced Monday it was dispatching around 15,000 troops, and Trump said he’d set up tent cities to house asylum seekers.

Trump has long called for an end to birthright citizenship, as have many conservatives. An executive order would spark an uphill legal battle for Trump about whether the president has the unilateral ability to declare that children born in the U.S. to those living here illegally aren’t citizens. Most scholars think he can’t.

Asked about the legality of such an executive order, Trump said, “they’re saying I can do it just with an executive order.” He added that “we’re the only country in the world where a person comes in and has a baby, and the baby is essentially a citizen of the United States.” A 2010 study from the Center for Immigration Studies, a group that supports immigration restrictions, showed that 30 countries offered birthright citizenship.

The Pew Research Center found in a survey published two years ago that births to “unauthorized immigrants” were declining and accounted for about 1 in 3 births to foreign-born mothers in the U.S. in 2014. About 275,000 babies were born to such parents in 2014, or about 7 percent of the 4 million births in the U.S. that year, according to Pew estimates based on government data. That represented a decline from 330,000 in 2009, at the end of the recession.

An excerpt of Trump’s interview was posted on Axios’ website on Tuesday.

The president said White House lawyers are reviewing his proposal. It’s unclear how quickly he would act and the White House did not provide further details.

Legal experts questioned whether Trump has the authority to do this by executive order.

Omar Jadwat, director of the Immigrants’ Rights Project at the American Civil Liberties Union in New York, said Tuesday that the Constitution is very clear.

“If you are born in the United States, you’re a citizen,” he said, adding that it was “outrageous that the president can think he can override constitutional guarantees by issuing an executive order,

Jadwat said the president has an obligation to uphold the Constitution. Trump can try to get Congress to pass a constitutional amendment, “but I don’t think they are anywhere close to getting that.”