President-elect Donald Trump's biggest promise was a large wall on the Mexican border, and besides already going back on his promise to not make taxpayers cover the costs, there may be environmental reasons Trump won't build his wall the way he wants to.

During an interview with The Golden State podcast this week, California's Lt. Gov. Gavin Newsom said there are state and federal environmental laws that could prevent Trump's wall from ever being finished.

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"There’s something called CEQA in California — NEPA at the federal level,” Newsom said, referring to the California Environmental Quality Act and the federal National Environmental Policy Act. “There’s indigenous lands and autonomies relating to governance on those lands. There are all kinds of obstructions as it relates to just getting zoning approval and getting building permits. All those things could be made very, very challenging for the administration.”

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Speaking to ATTN:, Newsom clarified that he's not arguing for using those laws himself, but rather noting Trump's wall is a "stupid idea" in part because others surely will.

"Of course the state has tools at its disposal, but it's not something I see as a particularly challenging issue, because it's simply never going to happen," Newsom said. "It's logistically impossible to build, and it’s a laughable proposal that Mexico is going to pay for it."

What the experts say

"It does seem clear that state environmental regimes will affect the accomplishment of such a massive federal endeavor," Jeffrey Romm, a professor of environmental science, policy and Management at the University of California, Berkeley, told ATTN:. Romm noted that "transboundary wildlife mobility, direct landscape transformations in construction and possible influences on water flows and qualities" will be environmental factors that could be the source of litigation and delay construction.

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Romm believes a variety of laws and international treaties will significantly impact the attempted building of a wall. "It would take years to get permits through state administrations, and these do not correspond among states," Romm said.

"Perhaps some monument-pieces of a visionary wall will be completed," Romm said, "but the whole thing would seem to be absolutely unattainable in decades without some overwhelming turn to centralized tyranny."

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Not everyone things building a wall will be so hard, however.

"I think Newsom is wrong," Holly Doremus, a professor of environmental regulation at the University of California, Berkeley, told ATTN:. "First off, CEQA doesn't apply to federal actions, so it would apply to California state and local government actions, but those aren't necessary to build a border wall. So that's out. Then he says NEPA. The reason that actually doesn't help is... congressional Republicans appear to be taking the position that [building the wall] was already authorized by a law that was passed in the George W. Bush era, which gives Homeland Security a great deal of authority to physically secure the border."

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Because of laws giving national security priority over the environment, the federal government may be able to overrule environmentally based objections to the wall.

The Supreme Court already rejected a constitutional challenge to building a fence along the border from environmental groups in 2008, so it's likely such a legal challenge wouldn't succeed now. But legal and regulatory challenges could delay construction, possibly for so long Trump is longer in office when rulings come down.