The idea has frequently been pushed by the most racist politicians in America throughout our history, both to keep the USA more white/European/Western/whatever, and as a political tactic to get people’s votes by inducing panic and fear of outsiders (even though there’s never been a real risk of hostile internal takeover). And it’s been quietly brewing again for the past few years. So it’s no surprise that the White House (which, again, includes a few white supremacists and several other highly racist individuals) would bring it up a week before midterm elections which could determine whether the Republican Party holds onto either chamber of Congress. Trump gets a partial win just by keeping us distracted and fueling the culture war, though the racist operatives in the White House probably are pushing for an Executive Order to undermine the 14th Amendment, no matter that it would be flagrantly unconstitutional. Case law and original intent are definitive about the meaning of the 14th’s clause “subject to the jurisdiction thereof.” It really was meant to be as widely interpretable as it seems, intending to undo the Dred Scott decision and ensure citizenship to the children of slaves—slaves, people who were taken from their own countries and held no intent to settle (nor allegiance toward!) the United States. “Illegal” immigrants are clearly covered, and it would require passing another Amendment to undo that. As James Ho cites in The Federalist (which I highly recommend reading in full, as it addresses a lot of little “but what if…”s):
Senator Edgar Cowan (R-PA) – who would later vote against the entire constitutional amendment anyway – was the first to speak in opposition to extending birthright citizenship to the children of foreigners. Cowan declared that, “if [a state] were overrun by another and a different race, it would have the right to absolutely expel them.” He feared that the Howard amendment would effectively deprive states of the authority to expel persons of different races – in particular, the Gypsies in his home state of Pennsylvania and the Chinese in California – by granting their children citizenship and thereby enabling foreign populations to overrun the country. Cowan objected especially to granting birthright citizenship to the children of aliens who “owe [the U.S.] no allegiance [and] who pretend to owe none,” and to those who regularly commit “trespass” within the U.S.[22]
In response, proponents of the Howard amendment endorsed Cowan’s interpretation. Senator John Conness (R-CA) responded specifically to Cowan’s concerns about extending birthright citizenship to the children of Chinese immigrants:
“The proposition before us … relates simply in that respect to the children begotten of Chinese parents in California, and it is proposed to declare that they shall be citizens. … I am in favor of doing so. … We are entirely ready to accept the provision proposed in this constitutional amendment, that the children born here of Mongolian parents shall be declared by the Constitution of the United States to be entitled to civil rights and to equal protection before the law with others.”