Article 2, Section I of the Constitution:

"No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States."

i'm sorry, but i don't care if people didn't like the guy...he was born here. if he hadn't been, this issue would've been brought up during the 7 years he was a state senator in Illinois or the 3ish years he served as a senator in DC representing the state. that's 10 full years he served publicly that his citizenship was never an issue and i can assure you...if it were actually a thing worth seriously considering, other people already looked into it way before this and found nothing to the contrary.