The North Portico of the White House is reflected in a mirror at sunrise on Christmas Day, the third day of the partial government shutdown now in its third week. (Oliver Contreras for The Washington Post)

White House officials on Wednesday asked federal agencies to send them a list of stalled services they would like to resume to minimize the public impact of the federal shutdown, three people familiar with the directive said.

The message, conveyed during a conference call between top officials at the White House Office of Management and Budget and federal agencies, is part of a broader effort to keep large parts of the federal government running while President Trump digs in for a lengthy battle over a border wall.

The Trump administration is directing federal employees, in many cases without pay, to process tax refunds, extend food-stamp benefits, provide documents for mortgage processing, keep a federal flood insurance program running, and open up national parks. Many of these activities had been prohibited during past shutdowns and were shuttered during the first two weeks of the current lapse.

The rapid and in many cases unprecedented scope of OMB’s directives has prompted criticism from Democrats and some Republicans that the White House is bending the rules to contain political fallout. During a funding lapse, agencies can retain certain employees to perform only essential functions that in many cases are matters of public health or national security.

It is very unusual for the White House to allow agencies to make so many changes to their contingency plans at this stage in a shutdown. The current lapse will be the longest in U.S. history if it lasts through Saturday, and White House officials said they are trying to provide maximum flexibility to blunt the impact on American families and businesses.

“OMB has routinely communicated with agencies throughout the partial shutdown to ensure we’re taking all steps to make this as painless as possible,” said Meghan Burris, an OMB spokeswoman. “Unlike previous administrations, we understand this is not easy on the American people, which is why we’re proactively identifying ways to keep the government running while remaining consistent with law.”

The effort by the OMB has been largely improvised and reactive, as few agencies had complete plans in place for an extended shutdown. Trump has said he will keep the government shutdown open for “months,” “years” or “whatever it takes” to get Democrats in Congress to sign off on $5.7 billion for building a wall along the U.S.-Mexico border.

“The reason you put out a plan before a shutdown starts is so that it’s clear these decisions are based on the law,” said Sam Berger, who was an OMB lawyer in the Obama administration during a shutdown in 2013.

Keeping basic functions of government intact during the shutdown has also taken the administration into uncharted legal territory, reversing precedents as it searches for ways to fund efforts for which Congress has not appropriated any money.

As the administration pushes more and more federal employees to work without pay, some unions are launching legal challenges.

The Antideficiency Act, passed more than 100 years ago, prohibits the government from spending money that hasn’t been appropriated by Congress. The law puts sharp constraints on what agencies do during a shutdown, because Congress hasn’t approved specific funding.

But its sweep is open to interpretation, and there have rarely been legal challenges during a shutdown to block certain programs or benefits from taking place. Still, violating the law can lead to fines or even criminal penalties, which has for years given federal employees pause.

Though Democrats have grumbled over the legally murky moves, they have not contested them broadly. Doing so would come at considerable political risk, putting the party in the untenable position of suing the administration to block tax refunds, food stamps and other popular services.

On the Wednesday call, OMB officials said they would review any requests from the agencies to see whether there was a legal way to allow specific programs to restart. They also signaled to agency officials they would continue doing it as much as possible and as quickly as possible, said the three people briefed on the call, who spoke on the condition of anonymity because they weren’t authorized to disclose the call’s contents.

The ad hoc nature of the restoration effort has left the administration with wide leeway to pick which functions are restored and which lapse along with their funding. Many agencies have scrapped or reversed key parts of their contingency plans because of blowback from businesses or interest groups, and some agencies have been slow to identify problems, in part because senior leaders left during the Christmas holiday and didn’t return until last week.

The review and response process is overseen largely by a small team at the OMB, led by acting director Russell Vought and his general counsel, Mark Paoletta. Many of the requests for changes came in late in the process because some agencies didn’t realize how many problems they had with their contingency plans until more than a week after the shutdown began on Dec. 22.

Last Friday, the Internal Revenue Service announced it would restart a program that provides income information for companies processing mortgage applications. On Sunday, the National Park Service brought some staff back, still with no pay, so that it could clean up areas at a number of national parks. On Monday, the White House announced the IRS would be allowed to pay tax refunds next month even if the shutdown continued, something that had previously been deemed to violate government rules. On Tuesday, the Agriculture Department said it would be able to pay food stamp benefits in February, even though it initially appeared to lack the funding to do so just a few days ago.

Collectively, these changes have served to extend government services to millions of people, even though in most cases the federal employees performing the functions will not receive any salary.

“This should be a serious challenge to the congressional leadership, because this sets up a presidency, that can — by whim of the OMB — pick and choose and give relief where they think it needs to be,” said Steve Bell, a Republican and former staff director of the Senate Budget Committee.

The OMB frequently has conference calls with federal agencies, and during the current lapse it has scheduled a large-scale call with numerous agencies roughly three times a week.

It is not unprecedented for White House officials to constantly monitor the impact of a government shutdown and make changes, particularly in the face of political pressure. During a funding lapse in 2013, the Obama administration allowed World War II veterans to visit the World War II Memorial, which as a national monument had been closed off.

But the Trump administration has gone much further, and the OMB call on Wednesday suggested that more changes could come.

Bell said by continuing operations that had previously been halted because employees weren’t allowed to work, the White House was effectively blunting some of the public relations backlash from people who are demanding their tax refunds or food stamp benefits.