Story highlights John McCain called Canada-born Ted Cruz's eligibility for the presidency a "legitimate question"

McCain himself was born outside the mainland United States but was the 2008 GOP nominee

Washington (CNN) Ted Cruz's explanation for why John McCain is questioning his eligibility to become president: McCain secretly supports Marco Rubio for the Republican nomination.

"Everybody knows John McCain is going to endorse Marco Rubio," Cruz told reporters Thursday when asked about how the Arizona senator had brought up whether his birthplace would make him ineligible for running for president.

"Their foreign policies are almost identical. Their immigration policies are identical," Cruz said. "So it's no surprise that people who are supporting other candidates in this race are going to jump on the silly Attacks that occur as we get closer and closer to the election."

Rubio has earned more establishment Republican support than Cruz, but McCain has not endorsed the Florida senator in the 2016 GOP presidential race -- instead holding off on supporting another candidate after his preference, South Carolina Sen. Lindsey Graham, dropped out.

Early Thursday evening, a McCain spokeswoman said he will not be endorsing a primary candidate at this point in the race.

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