The birthers say Rubio, a Florida Republican some in the GOP hope joins the 2012 ticket, isn't eligible to be president under Article 2 of the U.S. Constitution, which states "no person except a natural born citizen … shall be eligible" to be president or vice president, The St. Petersburg (Fla.) Times reported.
Rubio was born in Miami but birthers say he's not a "natural-born citizen," which the Constitution does not define, because he was not born to U.S. citizens.
The Times says the birthers cite writings from the time of America's formation and references in court cases since then.
Rubio was born in 1971 at Cedars of Lebanon Hospital in Miami, his office said, but his Cuban-born parents did not become citizens until 1975.
"Marco Rubio was born a Cuban citizen via his parents," said a headline on a blog by birther Charles Kerchner.
"We need the court to finally adjudicate this issue, who is a natural-born citizen," said birther Orly Taitz, a California dentist and lawyer.
Birthers cite an 1875 Supreme Court case, Minor vs. Happersett, in which the court used the term "natural-born citizen" to refer to people born in the United States born to U.S.-citizen parents.
"The arguments aren't crazy," said Georgetown law Professor Lawrence Solum. But, he added, "The much stronger argument suggests that if you were born on American soil that you would be considered a natural-born citizen."
Rubio's office says he considers himself a "natural-born" citizen.
"The price of our freedom and our liberty is that people can go out and spend a lot of time on stuff like this," he said. "For us, the more important thing is to focus on our job."