First we heard from Donald Trump a week ago who raised the question: Is Ted Cruz legally able to be President? At first, many people thought this was just another case of a competitor trying to cast doubt on Cruz's Presidential run.
Later in the week, Senator John McCain also raised doubts, saying Cruz should go before a court -- possibly the Supreme Court -- to determine if he is eligible. This could also be discounted by many people, since Senator McCain is no great fan of Cruz's.
However, over the weekend a more neutral expert weighed in -- Laurence Tribe of Harvard University, who is a noted Constitutional scholar. Professor Tribe said it was open to question as to whether Cruz would be eligible in that this particular clause in the Constitution has never been defined.
The phrasing in the Constitution that is in question is the requirement that a candidate for President must be a "naturally born citizen." In the 1700's, one legal definition said a naturally born citizen would be one whose parent are citizens and who was born in the country. By that definition, Cruz would not qualify, since he was born in Canada and his mother was a citizen but his father was not. Cruz actually was a dual citizen of the US and Canada until 2014.