Judge Andrew Napolitano broke down on Monday the legality of President Trump potentially declaring a national state of emergency in order to build his long-promised border wall.

Trump repeatedly has expressed the possibility of doing so, and said Sunday it would depend on "what's going to happen over the next few days."

He announced on Twitter Monday afternoon that he'd address the nation Tuesday night "on the Humanitarian and National Security crisis" at the border.

Napolitano, Fox News’ senior judicial analyst, said on "The Story" that in a time of true emergency, a president has valid emergency authorities.

"But he can't spend money and he can't take property unless the Congress has authorized it," he said. "That's directly from the Constitution."

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Napolitano referenced the National Emergencies Act of 1976, which he said defines an emergency as when the country is overwhelmed.

He said the emergency power is used to facilitate government assets in order to focus on an emergency, and not to give a president extra constitutional authority.

Napolitano added that potentially building a wall on private property would involve the government proving its case to do so in a trial.

"The president may be biting off more than he can chew here legally and financially. He is clearly in dangerous waters constitutionally," he said.

Watch Napolitano's discussion with Martha MacCallum above.

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