US president Donald Trump vowed to end the automatic right to citizenship for children born in the US to non-citizens.
Currently, all children born in the US get citizenship under the constitution.
"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," he said. "It’s ridiculous. And it has to end."
In fact a number of countries, including Canada and Mexico, offer birthright citizenship.
Mr Trump said that he could enact the change by signing an executive order but there are major doubts over whether this would be possible.
The legality of such a move, without explicit authorisation from Congress, is questionable, says the BBC’s North America reporter, Anthony Zurcher.
Coming just days before the mid-term elections, this could be viewed as another attempt by the president to rally his base and change focus to the immigration issue, which he views as a political winner, he adds.
It follows yesterday’s announcement that thousands of US troops would be sent to the southern border in response to the convoy of migrants that is inching northwards through Mexico.
But the group - known as the migrant caravan - isn’t expected to arrive at the border for several weeks.
Mr Trump’s critics said the move was a political statement ahead of the mid-terms.
Regardless, it is clear that immigration is an issue the president is keen to raise ahead of the crucial vote.