How would his Presidency be illegal if his citizenship isn't an issue?
I agree that, if he were ineligible to be President, then he would certainly have to leave office immediately. I don't even know how the government would go about untangling that mess, since there's no precedent for it. I doubt they would be willing to simply void everything he did the entire time, but I can see where if they didn't, it would be a feeding frenzy of lawsuits and court challenges.
I think what the voters think is VERY material. Right now, apparently, they aren't viewed by the courts as having standing to challenge a candidate's eligibility to run for office; only his opponents, apparently, are viewed as having standing. That seems ridiculous to me, since any voter in the US would qualify as an "involved party" in MY book.
Personally I think you should have to undergo a full Top Secret Clearance investigation going back to your childhood (and the results released to the public) to see if you have any ties to enemies of the state. Also ties with groups that want to overthrow the government should be considered and used as a disqualification. The constitution is too vague and this loop-hole allows placing the power of the White House in the hands of a plant funded by foreign groups. The havoc he or she could and has caused is unbelievable.
His birth certificate is a minor problem because I believe both of his parents were U.S. citizens. However there seems to be a problem with who his real father is. I believe this possibility is a source of embarrassment for him and is why he's hiding his original certificate. If he had a different father then the one listed on his phony replacement cert. I don't think that it would disqualify him, it would just be a minor issue that his supporters would simply rally behind.
Take for example that his real father is a Communist with a penchant for pole-smoking, who got high one night and inadvertently had a one-night-stand with his mother after a communist rally, I seriously doubt that Democrat voters would reject him. It's not his fault who his father was.
The fact that he's lying about it shows a weakness of character, but that has already been firmly established anyway through his conduct while in office. Democrat voters don't seem to shun candidates with weak character traits, they embrace them.