There are people who don’t believe that he was born in the United States (some think Kenya, some think Indonesia and others say Canada) and under the rules in place in 1961 his 18 year old mother was three months too young to qualify him for Citizenship At Birth. She would have had to be 19 years old at the time of his birth if he was born abroad.
That’s a legitimate legal/constitutional argument.
Speaking as a woman, that age thing seems terribly unfair. Was there ever a Supreme Court test case? Of course the men only was even more unfair, as a hundred years ago, paternity was a lot harder to prove than maternity before DNA testing existed.