– This already was going to be a challenging year to fill out federal tax returns after major tax-law changes took effect in 2018.

Then the political dispute over the proposed U.S.-Mexico border wall sent most Internal Revenue Service workers home last month.

Now, with the government partly shut down as tax-filing season approaches, there’s growing concern about whether a skeletal staff of IRS employees can handle the workload.

The Trump administration plans to call more IRS employees back to work — without pay — to process tax refunds.

Even if the shutdown ends soon, the damage to this tax season might already have taken place because of less training time for IRS employees, difficulty hiring seasonal workers to help process returns and a slowdown in getting critical IRS guidance to tax preparers.

“It’s the biggest tax reform change in 30 years. There are going to be many, many millions more questions that are asked. You’ve got a shutdown. You’ve got fewer employees,” said Tony Reardon, national president of the National Treasury Employees Union, which represents IRS workers.

“To me, that is all a big brew that spells potential trouble,” he said.

The IRS already has difficulties processing the roughly 150 million annual individual tax returns as the agency has been by hit by staff reductions and political controversy in recent years.

“Filing season is busy and compressed and challenging for everyone on a normal basis,” said Edward Karl, vice president of taxation for the American Institute of CPAs. “This will be a particularly challenging year.”

The IRS is among about a quarter of federal agencies whose funding lapsed on Dec. 22 after President Donald Trump and congressional Democrats couldn’t agree on appropriations because of Trump’s demand for $5.7 billion to fund the border wall. Trump had promised repeatedly during his presidential campaign that Mexico would pay for the wall.

About 70,000 IRS employees — roughly 88 percent of the workforce — have been furloughed, according to the IRS’ shutdown contingency plan. Issuing tax refunds is not among the agency tasks that would be allowed during a shutdown, which are limited by law to activities that are “necessary for the safety of human life or protection of government property,” the Nov. 29 contingency report said.

But an outcry about the effects of delaying issuing tax refunds led the Trump administration to declare last week it was changing course. The Office of Management and Budget cited a law that it said created a permanent appropriations for the payment of tax refunds.

“We are committed to ensuring that taxpayers receive their refunds notwithstanding the government shutdown,” IRS Commissioner Chuck Rettig said last week.

But, so far, no IRS employees have been called back to work, Reardon said.

“People are not happy, as you might imagine, about potentially going back to work without being paid,” he said. While being furloughed is difficult enough, going back to work requires spending money on work-related expenses while getting no paycheck.

“What does it take to get to work: fuel, child care?” he said. “All of these things cost money — money that you might not have.”

The union filed a lawsuit last week saying that forcing federal employees to work without pay violates the Constitution and that the Trump administration is using a too-broad definition of essential government services that requires working without pay.

The IRS will begin accepting tax returns on Jan. 28. Last year, the filing window opened on Jan. 29. By Feb. 3, about 18.3 million returns were submitted and most of those were processed. The IRS issued 6.2 million refunds in that first week.

People expecting refunds tend to file early, while those who have to pay taxes usually wait until closer to the April deadline.