President Donald Trump does not have the legal authority to declare a national emergency to build a border wall — and is biting "off more than he can chew" if he tries, Fox News analyst and retired Judge Andrew Napolitano said Monday.

In remarks on Fox News' "The Story With Martha MacCallum," Napolitano warned of "dangerous constitutional waters" if the president makes good on his suggestion he would declare an national emergency to bypass Congress and build the border wall.

In a time of true emergency, a president has valid emergency authorities, Napolitano asserted.

"The numbers [on border crossings] are very impressive, but an emergency is defined . . . as when the government is overwhelmed and its ordinary assets don't work," he said. "I don't think President Trump would say that."

"[H]e can't spend money and he can't take property unless the Congress has authorized it," he added. "That's directly from the Constitution."

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

In those cases, he pointed out, 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 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 said. "He is clearly in dangerous waters constitutionally."

Trump has asked for more than $5 billion to build a wall; Democrats have offered $1.3 billion for border security and no money for the wall. The standoff has triggered a partial government shutdown.

Trump is set to deliver a primetime address about the situation Tuesday, and will travel to the border Thursday.