But he's not skipping Congress to change the Constitution. He cannot constitutionally do that. A constitutional change in the US requires both houses and all 50 states. The 14th amendment will be unchanged regardless of what he does.
What he is doing is implementing legislation to interpret and enforce the 14th amendment in a certain way. He knows that even many Republicans reject that interpretation so the legislation would probably fail, esp. if they lose a house next week (yes, it's that close). Hence the executive order.
But, again, the 14th amendment will not change. What will change is how the government interprets and enforces it.
A constitutional amendment actually requires a two-thirds vote of both houses of Congress but only three quarters of the states, not all fifty - thus thirty eight states.
Article 5 of the United States Constitution said:
The Congress, whenever two thirds of both houses shall deem it necessary, shall propose amendments to this Constitution, or, on the application of the legislatures of two thirds of the several states, shall call a convention for proposing amendments, which, in either case, shall be valid to all intents and purposes, as part of this Constitution, when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof, as the one or the other mode of ratification may be proposed by the Congress ...
And Trump isn't proposing legislation. That would take the fight out of his hands and he'd never get it passed anyway because even a lot of Republicans wouldn't go for that. He's quite serious about an Executive Order. For now he hopes that the threat of an Executive Order on this subject will rile up his base to get out and vote Republican next Tuesday. But it wouldn't surprise me at all if it becomes more than a threat. He could well issue an Executive Order eliminating birthright citizenship sometime after the election. It would then be immediately challenged in court. It would almost certainly be declared unconstitutional by whatever judge heard the case first. It would start working its way up the appellate system, almost certainly being declared unconstitutional again and again - once more riling up his base to a fever pitch, allowing him to denounce the "activist" judges, setting him up for his bid for re-election in 2020. Mark my words.
There are some (a small minority) of legal scholars who believe that Trump could act on this by Executive Order. They argue that while he can't amend the constitution, this wouldn't be an
amendment to the Constitution but an
interpretation of the Constitution. Article 14 says: "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States ..." The key phrase they'd argue is "... subject to the jurisdiction thereof ..."
The argument is that as children, and as the children of undocumented immigrants, they aren't "subject to the jurisdiction of the [United States.]" I think it's a flimsy and ridiculous argument. If you're in the United States you're subject to the jurisdiction of the United States. Unless you're a diplomat. I suspect that's what the amendment was meant to address - children born to foreign diplomats even in the 1860's when Article 14 was approved. Foreign diplomats aren't subject to the jurisdiction of the host country, and I suspect that the amendment was meant to say that a child born to a foreign diplomat was not subject to US jurisdiction, and therefore would not be a citizen of the US. But undocumented immigrants are subject to US jurisdiction (otherwise, they couldn't be deported) and so, logically, so are their children. So that argument fails.
Some have argued that Article 14 needs to be interpreted in context: in 1868 when it was approved it was meant to defend the voting rights of former black slaves. Therefore it should be read only in that context. But the Supreme Court long ago rejected that argument. In an 1898 case, the Supreme Court ruled that Article 14 applied to a man born in San Francisco to Chinese parents who were not American citizens. Chinese immigrants at the time were not eligible for American citizenship. But the Supreme Court said in that case that their children, if born in the United States, were indeed American citizens.
The vast majority (the overwhelming consensus) of legal scholars say that Trump can't eliminate birthright citizenship by Executive Order. But he doesn't want to eliminate it. He wants to be seen putting up a fight to eliminate it - and those are a couple of the arguments that he and his cult will put forward.