In his motion to dismiss the complaint on Monday, Rubio's lawyers argued that the Florida senator is a "natural born citizen," a requirement to become president. The motion also notes that the complaint filed against Rubio and Cruz "would jeopardize centuries of precedent and deem at least six former presidents ineligible for office."
Rubio was born in the United States, but his parents did not become American citizens until after his birth. And some birthers argue that he is not a "natural born citizen" because his parents were not citizens.
Cruz has faced greater scrutiny because he was born in Canada. His mother was an American citizen, so he became a United States citizen at birth. But some legal experts point out that the Supreme Court has not clearly defined a "natural born citizen," leaving room for the court to decide that Cruz does not qualify.
H/t The Hill