Retiring U.S. House Speaker Paul Ryan (R-WI) is pushing back against President Donald Trump's threat to end birthright citizenship with an executive order.

Trump "obviously cannot do that," Ryan told Kentucky talk radio station WVLK in an interview on Tuesday.

“You cannot end birthright citizenship with an executive order," Ryan said. "As a conservative, I’m a believer in following the plain text of the Constitution, and I think in this case the 14th Amendment is pretty clear, and that would involve a very, very lengthy constitutional process. But where we obviously totally agree with the president is getting at the root issue here, which is unchecked illegal immigration.”

On Wednesday, Trump fired back at Ryan, whom the president thinks "should be "focusing on holding the majority rather than giving his opinions on Birthright Citizenship, something he knows nothing about!"

Those that oppose the president begrudgingly found themselves coming to Ryan's defense, but they made some good points.

Ryan certainly knows more about it than Trump.

Others pointed out the president's hypocrisy on how he approaches constitutional amendments; namely, why is the 14th Amendment subject to change while the 2nd isn't?

Trump, as president, does not have the power to change the Constitution.

Though one person asked an awkward question:

Also breaking from the president, albeit less blatantly, was Senate Majority Whip John Cornyn (R-TX), who said birthright citizenship is "a symptom of a bigger problem. And my position on immigration is pretty simple: legal immigration is good, illegal immigration is bad."

Cornyn believes the solution lies with Congress.

"We need less posturing and less rhetoric on this and more solutions," Cornyn said. "I know the president is enormously frustrated, and I am frustrated too, about our inability to work together on a bipartisan basis to solve the underlying problem, but that is what I think we have to do."

On Tuesday, Trump boasted that he alone could issue an executive order ending birthright citizenship because "they're saying" he can.

"It was always told to me that you needed a constitutional amendment. Guess what? You don't," Trump told Axios. "You can definitely do it with an Act of Congress," he added, "but now they're saying I can do it just with an executive order."

Trump did not specify who told him he could do that right before he falsely claimed the United States is the only country that offers citizenship to babies born on its soil.

"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 ... with all of those benefits," Trump continued. "It's ridiculous. It's ridiculous. And it has to end."

In fact, more than 30 countries offer some form of birthright citizenship. And precedent is not on president's side.

The 14th Amendment was ratified on July 9, 1868. It reads: "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 in 1898 ruled in United States v. Wong Kim Ark that anyone born in the United States, regardless of their parents' immigration status, shall be granted American citizenship. This landmark decision has set a legal precedent for more than a century.

"The 14th Amendment's citizenship guarantee is clear," according to Omar Jadwat, director of the ACLU's Immigrants' Rights Project. "This is a transparent and blatantly unconstitutional attempt to sow division and fan the flames of anti-immigrant hatred in the days ahead of the midterms."

But not everyone agrees.

John Eastman, a constitutional scholar and director of Chapman University's Center for Constitutional Jurisprudence, said the language of the 14th Amendment is key. He told Axios that "subject to the jurisdiction thereof" means citizenship only for people with green cards or legal residency.

Either way, if Trump does issue such an executive order, "the courts would have to weigh in in a way they haven't," Eastman said.

Senator Lindsey Graham (R-SC), who has recently forged a political alliance with Trump, plans on introducing legislation to end birthright citizenship, which he called "absurd policy."

"This policy is a magnet for illegal immigration out of the mainstream of the developed world,"  Graham said earlier this week, "and needs to come to an end."