He is being protected because of circumstance and that he is useful.
It is also probable that a few Republicans have checked the issue out, but there is too little evidence to be able to bring out more than a few accusations.
Once Obama is out of office, his usefulness will cease and so will his protection. what isn’t possible now will be doable in the future.
However, attacking his insane policies now is very much doable. I’d rather have him gone based on his public record, rather than him gone, because we found the faintest of provable documentation about his ineligibility.