According to Art 2 of the Constitution, the President must be a natural born citizen. Because Obama's father was never a US citizen, it's impossible for him to be natural born, even if he was born in Hawaii.
The Civil Right's Act states "That all persons born in the United States and not subject to any foreign power, excluding Indians not taxed, are hereby declared to be citizens of the United States"
Obama admits himself that he was born a British subject, but lost it at adulthood, so this proves he can't be natural born. These facts are validated by Supreme Court Case Minor v. Happersett which states "it was never doubted that all children born in a country of parents who were its citizens became themselves, upon their birth, citizens also. These were natives, or natural-born citizens"