Representatives from the Hopi, Zuni, Ute Mountain Ute, the Navajo Nation and the Ute Indian tribes accused Trump of exceeding "the limited authority delegated to his office," violating "the Antiquities Act and the separation of powers established in the Constitution" and circumventing the law by "attempting to evade that strict limitation" of his power.
The lawsuit, the first against Trump's decision to shrink the monument, comes at the same time that a group of environmental and conservation organizations sued Trump and Interior Secretary Ryan Zinke for dramatically shrinking Utah's Grand Staircase-Escalante National Monument.
That lawsuit, which comprises The Wilderness Society, the Natural Resources Defense Council, the Sierra Club and seven other groups as plaintiffs, argues that Trump's decision to reduce the size of Grand Staircase-Escalante was "unlawful" and "exceeds his authority under the US Constitution and the Antiquities Act."
The White House did not immediately respond to a request for comment. Heather Swift, a spokeswoman for the Interior Department, said: "We are well within our authority," and referred further questions to the Department of Justice.
Trump had announced on Monday in Salt Lake City that he would follow Zinke's recommendations and reduce Bears Ears National Monument by over 80% and Grand Staircase-Escalante National Monument by roughly 45%. The two presidential proclamations, which Trump signed at an event at the Utah Capitol, would break Bears Ears into two national monuments and Grand Staircase-Escalante into three separate monuments.
The lawsuit against Trump's action on Bears Ears argues that Trump is not shrinking the national monument, but revoking its status all together.
"The President was plainly aware that he lacked the authority to revoke a monument and is thus transparently attempting to evade that strict limitation by purporting to reduce it but, as described herein, the President's action must be viewed as a revocation, particularly with respect to all objects not included in the two 'new' monuments," reads the lawsuit.
Trump said Monday his decision was made to "reverse federal overreach and restore the rights of this land to your citizens."
"The families and communities of Utah know and love this land the best, and you know the best how to take care of your land. You know how to protect it. And you know best how to conserve this land for many, many generations to come," the President said.
Largely ignoring the tribes who oppose the decision, Trump framed the fight as one between local officials and bureaucrats in Washington, who he said "don't care for your land like you do."
Like the Bears Ears lawsuit, the filing against Trump's actions on Grand Staircase-Escalante argues Trump has exceeded his authority.
"The Act authorizes Presidents to create national monuments; it does not authorize Presidents to abolish them either in whole or in part, as President Trump's action attempts to do," reads the filing.
The Antiquities Act, a presidential power that was first signed by President Teddy Roosevelt in 1906, gives the President the power only to designate a monument, not revoke it, the plaintiffs argue. Additionally, the lawsuit argues that Trump's order violates the separation of powers because Congress has the right to change national monuments created under the Antiquities Act.
"Grand Staircase is a cradle of biodiversity, and losing even an acre would be a crime," said Taylor McKinnon of the Center for Biological Diversity, a plaintiff in the case. "We must protect this monument's wildlife, stunning landscapes and cultural treasures for future generations. Trump and the fossil-fuel industry have picked the wrong battle."
Though environmental groups have also protested Trump's decision on Bears Ears and plan to file their own suits, it is intentional that the five tribes filed the first lawsuit against the Trump administration for that monument.
"The tribes feel it was important to file first, to be ahead of the line, to make it very clear that this is not just a conservation issue," said Natalie Landreth, a senior staff attorney at the Native American Rights Fund, a group representing the Hopi, Zuni and Ute Mountain Ute in the lawsuit. "To them, it is a tribal sovereignty issue."
Trump administration officials have long expected that Trump's proclamations would end up in court, given that environmental groups and tribes had pledged to do so.
Zinke, speaking with reporters aboard Air Force One on Monday before the lawsuit was filed, accused "special interests" of polarizing the issue and said he isn't going to shy away from any legal challenge.
"I don't yield to pressure, only higher principle. And I don't think public policy should be based on the threat of lawsuit," he said.
He added that the changes have been revised by Interior Department solicitors and approved.
"I feel confident in the Department of Justice," Zinke added. "And I feel confident that we will prevail because it's the right thing to do."