Donald Trump made news in the last few weeks when he raised the question of whether Ted Cruz could run for President. The Constitution requires that someone be a "natural born citizen" -- but it doesn't define that term!
If "natural born citizen" means someone with at least one American parent, born anywhere in the world, then Ted Cruz qualifies. If "natural born citizen" means someone with two American parents, born in the United States, then Ted Cruz does not qualify.
Now a new challenge has been raised -- and this time it is to Marco Rubio. Marco was born in the United States, but, at the time he was born, neither of his parents were American citizens. They became American citizens after he was born, and he thus became a US citizen.
The lawsuit against Rubio, as reported by the Tampa Bay Times, alleges that Rubio is not eligible to run for President because he is not a "natural born citizen." So the Republican Party is in the unusual situation of having two of its top Presidential candidates, with Cuban parents, being potentially challenged with their citizenship!