WASHINGTON (AP) — Democratic senators on Thursday criticized a National Park Service plan to impose steep increases in entrance fees at 17 of its most popular parks, including the Grand Canyon, Yosemite, Yellowstone and Zion, calling the proposed rates "arbitrary and unjustifiable."

Under a plan announced this week, visitors to many national parks would be charged $70 per vehicle during the peak summer season, up from $25 or $30 per vehicle now. Officials say the higher fees are needed to address an $11 billion backlog of maintenance and infrastructure projects that have been put off for years.

Senators said the plan would exclude many Americans from enjoying national parks. Sen. Maria Cantwell of Washington state and other Democrats said the proposal is especially egregious because the Trump administration is recommending severe budget cuts for the park service.

Claims that the increased fees are needed to reduce the maintenance backlog are "undercut by the administration's budget proposal to cut" park service operations by $200 million, including a $93 million cut to facility operations and maintenance, the senators wrote in a letter to Interior Secretary Ryan Zinke.

"If implemented, your proposals to increase fees while cutting agency funding would serve to shift major costs to park visitors and undermine public access to national parks — actions that would be a disservice to the American people," they said.

Cantwell, the senior Democrat on the Senate Energy and Natural Resources Committee, organized the letter. It also is signed by Democratic Sens. Patty Murray of Washington state, Kamala Harris and Dianne Feinstein of California, Ron Wyden of Oregon, Mazie Hirono of Hawaii, Tom Udall and Martin Heinrich of New Mexico and Tim Kaine of Virginia, as well as independent Sen. Bernie Sanders of Vermont.

Theresa Pierno, president and CEO of the National Parks Conservation Association, an advocacy group that works to protect parks, said the proposed increase could make the Grand Canyon and other popular parks unaffordable for many families.

"The solution to our parks' repair needs cannot and should not be largely shouldered by its visitors," she said. "If the (Trump) administration wants to support national parks, it needs to walk the walk and work with Congress to address the maintenance backlog" through the budget process.

A 30-day public comment period opened Tuesday. The park service said it expects to raise $70 million a year with the proposal at a time when national parks repeatedly have been breaking visitation records and putting a strain on park resources. Nearly 6 million people visited the Grand Canyon last year.

Zinke said the plan was part of his "vision to look at the future of our parks and take action in order to ensure that our grandkids' grandkids will have the same if not better experience than we have today."

Annual $80 passes for federal lands would not change, though fees would go up for pedestrians and motorcyclists. The higher fees would apply only during the five busiest contiguous months for parks. For most that's May through September when many families are on vacation.

The proposal would not affect several free weekends and holidays.