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"I ran against the guy, so of course there's things that I disagree with him on," Christie told ABC's George Stephanopoulos in an exclusive interview Sunday on "This Week".
Christie reiterated he was supporting Trump because he believes the real estate mogul is the only candidate who can defeat Democratic front-runner Hillary Clinton in the general election.
"This is now a choice, and of the candidates remaining on that stage, he is the best person to beat Hillary Clinton, which is job one for Republicans," Christie said. "He is the best person to beat her and he is the best person to help keep America safe."
When asked how Trump would get Mexico to pay for building a border wall with the U.S., one of his signature policy issues, Christie would not say how Trump would ensure the wall is built.
"This is the thing that I said and he will answer that question," Christie said. "But you know, to me, I'm not answering the question for him this morning."
Christie added Trump is being held to a higher standard than his rivals.
"Some members of the media and others want to hold Mr. Trump to a different standard than they want to hold the other candidates to," he said.
In a later interview on “This Week,” South Carolina Gov. Nikki Haley, who endorsed Florida Sen. Marco Rubio before Trump won her state's Republican primary, said she does not understand why Christie would endorse Trump.
“Chris is a dear friend but none of us understands why he did this," she said.