1. Let us assume he is not a natural born citizen.
2. Then he is:
3. Either an illegal immigrant (and an alien)
4. or a legal immigrant (and an alien)
5. or a naturalized citizen.
6. But he has a U.S. State department issued passport
7. So he is not 3 or 4.
8. If he is 5, there would be naturalization records and it would be easily accessible. None are known.
9. He has to be natural born. Otherwise, US State department is dumb as doornail to issue a passport to a foreigner. Also, the State of Illinois is careless enough to elect a foreigner for State & federal Senate. In which case, why bother to even worry? what is there to defend?