(CNN) -- John Bolton, the former U.S. permanent representative to the United Nations, endorsed presidential candidate Mitt Romney.
Bolton made the endorsement Wednesday on Fox News Channel, where he is a paid contributor.
"I followed the ... William F. Buckley Jr. test: find the most conservative candidate capable of getting elected, and there are obviously two variables there," Bolton said on Fox News' "On the Record w/ Greta Van Susteren."
"In the words of the old song 'Give Me That Old Time Religion,' Romney is conservative enough and I think that's critical."
Bolton served in the administrations of former presidents, Ronald Reagan and George H.W. Bush, before former President George W. Bush appointed him to the U.N. role.
He briefly flirted with a presidential run in 2011, and endorsed Sen. John McCain's presidential bid in February 2008.