If he haa a valid U.S. Passport then he’s a U.S. Citizen. Under federal law, a passport is considered primary evidence of citizenship and identity.
https://www.law.cornell.edu/cfr/text/42/435.407
The validity of a U.S. Passport is only as good as the documentation presented to obtain it. Since Cruz birth records are sealed, we don’t know what he used to obtain a passport, or if it’s legit. We should rather know before he’s the nominee, not after.