As others have said, since we have never had a non human president, we will never know.
That is not true. We do know what would happen, because we know that the United States is governed by the Constitution, and the Constitution is very clear on what the parameters for removing a president from office are. That's the whole
point of having a Constitution. It's a set of guidelines that tell you
in advance how to deal with problems that may arise. So it's nonsense to say "we can't know what would happen." All we have to do is consult the Constitution and the law, and those codified, systematic guidelines allow us to predict what would happen. The Constitution does not require a president to be human. A nonhuman absolutely could be president, as long as he, she, or it met the legal definition of a person, was born within the United States, was at least 35 years old, and had resided within the US for at least 14 years.
It's certainly possible that the voters might be unwilling to elect a nonhuman president, or to re-elect a president who somehow "turned" nonhuman during his or her term, but there is absolutely no legal way to remove a sitting president from office unless he or she is convicted of high crimes or misdemeanors, or is rendered incompetent to fulfill the duties of the office. Certainly if the president were mutated, there could be those who would attempt to argue that the mutation rendered him or her incompetent, but so long as the president retained his or her intelligence and reason, that argument would be difficult to prove. It might lead to a Supreme Court challenge, maybe an attempt to pass laws defining a "person" as a human being only, but there's no guarantee that such laws would be passed or would hold up to a Supreme Court challenge if they were passed. It would depend on a variety of factors, including just how pervasive the prejudice against mutants (or other nonhuman sophonts) was in the society, how powerful the President's political opposition was, and the like.
But I'm willing to bet that if the President shows up at the White House with blue fur and able to lift a car, it wouldn't be business as usual for him.
It would obviously create controversy, but it would just as obviously have no effect on the President's Constitutionally defined eligibility for the office. Remember, as I've said already, many presidents have been wildly unpopular or hated while in office, yet have still managed to complete their terms.
I still stand by my original point that Magneto's plan wasn't very well thought out, and wouldn't benefit him or other mutants in any way. It would make them society's number one enemy.
And the mistake you're making is assuming that Magneto's
professed goal was his
actual goal. The stuff about bringing enlightenment and tolerance to the world's leaders was just his rhetoric to justify his actions. At the core, what he wanted was to punish them and inflict poetic justice upon them -- to make them suffer the same persecution and exclusion they'd inflicted on him and people like him. Given that that was his true purpose, the plan was about as well thought out as it needed to be -- aside from the error that resulted in the field actually killing humans rather than simply mutating them.
In theory, the John Bryne Post-Crisis Superman could have run for president since he was technically born in the US.
Yes, they actually did a big crossover event (the name slips my mind) that showed the possible futures of various superheroes, and the one for Superman showed him becoming the US President, with this exact justification being used (that his birthing matrix didn't decant him until he arrived in Kansas so he was technically a US native).