Speaking to reporters outside the White House on Wednesday, President Trump again threatened to end what he called the "ridiculous" policy of birthright citizenship, which awards citizenship automatically to those born in the United States.

"We're looking at that very seriously," Trump told reporters as he left the White House for Kentucky. "Birthright citizenship, where you have a baby on our land — you walk over the border, have a baby, congratulations, the baby's now a U.S. citizen. We're looking at it very, very seriously ...It’s, frankly, ridiculous."

During his campaign and again last year, Trump suggested he might use an executive order to end birthright citizenship, saying it encourages illegal immigration and "anchor babies." Democrats, who command an overwhelming share of the immigrant vote, immediately called the idea unconstitutional.

While illegal immigrants cannot vote, under the birthright citizenship policy their children can.

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The president's announcement came hours after the White House said it would move to scrap a major court agreement in order to allow for migrant families to be detained longer as their cases are being considered, instead of having to release them after 20 days.

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Legal experts say the ultimate question regarding birthright citizenship is whether the 14th Amendment – which affords citizenship to "[a]ll 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" – encompasses the children of illegal immigrants.

The unresolved legal dispute centers on whether those children are "subject" to the jurisdiction of the United States.

In an op-ed published last year by FoxNews.com, Heritage Foundation senior legal fellow Hans A. von Spakovsky said critics "conveniently ignored or misinterpreted" the 14th Amendment's requirement that illegal immigrant children be "subject to" the jurisdiction of the United States.

"The fact that tourists or illegal immigrants are subject to our laws and our courts if they violate our laws means that they are subject to the territorial jurisdiction of the U.S. and can be prosecuted," Spakovsky wrote. "But it does not place them within the political 'jurisdiction' of the United States as that phrase was defined by the framers of the 14th Amendment."

Spakovsky added: "Today many people do not seem to understand the distinction between partial, territorial jurisdiction – which subjects all foreigners who enter the U.S. to the jurisdiction of our laws – and complete political jurisdiction, which requires allegiance to the U.S. government as well."

Michael Anton, a former national security adviser for Trump, also told Fox News last July that "there’s a clause in the middle of the amendment that people ignore or they misinterpret – subject to the jurisdiction thereof.”

"What they are saying is, if you are born on U.S. soil subject to the jurisdiction of the United States — meaning you’re the child of citizens or the child of legal immigrants, then you are entitled to citizenship,” Anton told Fox News’ Tucker Carlson. “If you are here illegally, if you owe allegiance to a foreign nation, if you’re the citizen of a foreign country, that clause does not apply to you.”

The president first took aim at birthright citizenship in an October 2018 interview with Axios.

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“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 said. “It’s ridiculous. It’s ridiculous. And it has to end.”

Fox News' Ronn Blitzer contributed to this report.