Article II, Section 1, paragraph 5 of the U.S. Constitution provides qualifications for becoming the president of the United States: “No person except a natural born citizen, or a citizen of the United States … shall be eligible to the office of President.”
Sen. John McCain, R-Ariz., qualifies to run for and be president of the United States because his parents are citizens, thereby making him a citizen of the United States. Period.
Barack Obama stands as president of the United States because his mother is a citizen of the United States. And so is he. And in addition President Obama is a natural born citizen of the United States. Period.
Any person who has one parent who is a citizen attains citizenship due to the fact that her/his parent is a citizen. Period.