Former President Barack Obama spoke out Friday against President Donald Trump’s continued claims that he can end birthright citizenship for children born in the U.S. to parents who aren’t citizens.

“A president doesn’t get to decide on his own who’s an American citizen and who’s not,” Obama told a crowd in Florida while stumping for two Democratic candidates there: Andrew Gillum, the mayor of Tallahassee who is running for governor, and Sen. Bill Nelson, who is seeking a fourth term.

“That’s not how the Constitution of the United States works,” added Obama, who once taught constitutional law at the University of Chicago. “That’s not how the Bill of Rights works. That’s not how our democracy works.”

Obama is the latest person to strike down Trump’s repeated claims that he can end birthright citizenship, a right enshrined under the 14th Amendment.

“The Constitution does not — I say that to the media — does not require it. Read it. Because illegal aliens are not subject to the jurisdiction of the United States,” Trump insisted Wednesday at a rally in Florida, doubling down on comments he made to Axios earlier in the week.

“We will keep the criminals, the drug dealers, we will keep them all out of our country,” he added. “We will get rid of all of this. We will end finally birthright citizenship.”

House Speaker Paul Ryan (R-Wis.) has also challenged Trump’s claims, telling Kentucky’s WVLK radio station on Tuesday that the president “obviously cannot do that.”

“You know, as a conservative, I’m a believer in following the plain text of the Constitution,” Ryan said, “and I think in this case the 14th Amendment is pretty clear, and that would involve a very, very lengthy constitutional process.”