Republican Sen. Steve Daines of Montana warned Interior Secretary David Bernhardt on Saturday of "significant risk to property and public health" without funding.
Parks were closed during the 2013 government shutdown.
"We are taking this extraordinary step to ensure that parks are protected, and that visitors can continue to access parks with limited basic services", Smith said.
Unlike most previous government shutdowns, the Trump administration has gone to great lengths to keep parks open despite concerns from groups that it leaves national treasures vulnerable to vandalism and creates a unsafe environment for visitors.
"It's hard to see why allowing parks to tap fee revenue to keep popular parks open during a government shutdown would be anything other than a sensible policy - and one that is largely consistent with the origins of the current National Park Service fee program", Regan said. The Federal Lands Recreation Enhancement Act (FLREA) stipulates that fees charged by federal parks are created to fund services for visitors - not the maintenance of parks.
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Incoming chairwoman of the House Appropriations Interior-Environment subcommittee Rep. Betty McCollum, D-Minnesota, suggested the move is likely illegal.
He explained, "the only time the park felt close to getting out-of-hand was during peak visitation over the holidays due to the sheer number of visitors, and fortunately, the amount of damage was minimal".
"This is an obligation of government", she said.
House Natural Resources Committee Chairman Raul Grijalva, an Arizona Democrat, issued a statement as well on Sunday slamming the administration's decision and saying the committee would demand answers about the legal justification behind the move.
"Either way, this president is only happy as long as the American people pay for his every whim whenever it suits him, " Grijalva said.
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The Park Service is part of the Department of the Interior, one of the agencies affected by the prolonged partial government shutdown that began in late December. According to official estimates, as many as 16,000 of the Park Service's 20,000-person winter workforce are furloughed. Trump said Friday that it could last "months or even years".
"The NPS now has funds derived from entrance, camping, parking and other fees collected from park visitors that would typically be used for future projects at parks".
Fees collected at parks are supposed to enhance the visitor experience, which can include films about a site's resources and restoration projects that are aimed at activities such as hunting, fishing and photography.
This weekend, NPS Deputy Director P. Daniel Smith announced the agency will use internal funds to address the growing sanitation and safety concerns plaguing national parks. According to the Washington Post, a team of 40 volunteers have been cleaning toilets, restocking toilet paper, and recovering trash throughout Joshua Tree National Park.
Mesa Verde reported $65.7 million in maintenance needs that have been delayed because of budget constraints.
"Relaxing those internal controls, as the Park Service appears to be doing, is a positive step toward better addressing the agency's most pressing maintenance and operational challenges", Regan said.More news: Camp Fire: Trump says he's told FEMA to halt relief money