The threat of a partial government shutdown this week comes as several federal agencies are still battling to dig out from pandemic backlogs.

       The Internal Revenue Service is struggling to serve taxpayers 18 months after the pandemic sidelined thousands of employees. Close to a half-million immigrants are on a State Department list to schedule interviews for their visa applications — and the wait for a passport is now as long as 16 weeks. Thousands of documents for Social Security benefits lay unprocessed this summer in field offices where in-person service has been suspended since March 2020, the agency’s watchdog recently found.

       A shutdown would magnify these delays and further weaken services that the pandemic has hobbled while many federal offices have remained closed or with limited operations during the public health crisis, employees and experts on the federal government said Tuesday — even if the closure were brief.

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       The Biden administration says it would ensure that if government funding expires at midnight Thursday, federal employees involved in the pandemic response would stay on the job.

       But a shutdown would furlough hundreds of thousands of workers — potentially far more than during the record-setting but partial 35-day shutdown during the Trump administration in late 2018 and 2019. And some day-to-day services would be crippled at agencies already struggling to stay afloat.

       “It’s burning down the house in the middle of a blizzard,” said Max Stier, president and chief executive of the nonpartisan Partnership for Public Service. “It’s going to make what’s hard even harder.”

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       The IRS’s contingency plan, for example, would recall about 32,000 of the agency’s 82,000 employees to perform “essential” roles that include computer operations, mail processing, and building and data security.

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       But tax audits, collection activity, customer service phone lines, tax return processing and other operations would cease. The IRS is already months behind on tax refunds, audits and other services that were delayed early in the coronavirus pandemic as thousands of employees were sent home to work and could not answer phones or access sensitive taxpayer data.

       Even now, a notice on the agency’s website warns taxpayers that while “we’re open and processing mail, tax returns, payments, refunds and correspondence,” the pandemic “continues to cause delays in some of our services.”

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       The list of delays includes live phone support, processing of paper tax returns, answering mail from taxpayers and reviews of tax returns — even electronic ones. That’s a lot of the IRS’s core mission.

       “IRS employees have made great progress on clearing backlogs that developed at the start of the pandemic, even while delivering successful filing seasons and handling the increased workload of administering various economic relief programs,” Tony Reardon, national president of the National Treasury Employees Union, said in a statement.

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       He called any unpaid furloughs during a shutdown “a setback on that progress.”

       Federal agencies are still updating their contingency plans for a lapse in appropriations, officials said, and any furloughs or requirement to stay on the job would also apply to teleworking employees.

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       “We fully expect Congress to work in a bipartisan fashion to keep our government open, get disaster relief to the Americans who need it, and avoid a catastrophic default, especially as we continue to confront the pandemic and power an economic recovery,” Abdullah Hasan, a spokesman for the White House budget office, said in a statement.

       It’s unclear how a shutdown might affect President Biden’s vaccine mandate for federal workers, which has a deadline of Nov. 22. Managers and union officials across the government aren’t yet sure whether furloughed employees would get the four hours of leave they’re entitled to when they get shots.

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       “They’re very serious about these deadlines,” said Jacqueline Simon, national policy director for the American Federation of Government Employees, the largest union representing federal workers.

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       Simon blamed any pandemic-related delays to agency operations on staffing cuts during the Trump administration, which hollowed out many agencies as career staff quit or retired and were not replaced.

       “I’m not familiar with an agency that hasn’t been able to meet its mission” during the pandemic, she said.

       The Labor Department’s internal watchdog found in a report issued Tuesday, though, that productivity at the federal worker compensation program fell during the pandemic. Claims have slowed by 15 percent, the inspector general’s office found, a decline driven largely by a growth in covid-19 claims that take longer to process.

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       While the Office of Workers’ Compensation Programs has sped up the process in recent months, 46 percent of covid-19 claims remained open at the end of March, auditors found.

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       Most of the staff in the office would be furloughed during a government shutdown, according to the Labor Department’s contingency plan.

       The Social Security Administration, meanwhile, has been under pressure for months from lawmakers on Capitol Hill to return to serving the public in person after complaints from constituents who do not have access to the Internet.

       Unlike the IRS, the vast majority of the agency’s workforce would be considered essential and kept at work during a shutdown, according to contingency plans. But its national network of 1,200 field offices, which provide vital face-to-face services to low-income applicants for disability benefits, would remain open only for “dire need” appointments. Applications for benefits and other front-line activities would continue during a shutdown, according to the agency’s contingency plan.

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       “The agency makes a point of saying they’re open for business, but you still can’t walk into a field office,” said Nancy Altman, president of Social Security Works, an advocacy group.

       The State Department’s Bureau of Consular Affairs is upfront about its pandemic-related delays to service.

       “We have taken several steps to address these challenges, including ensuring all our employees have access to vaccines, bringing back staff at our agencies, instituting overtime, temporarily assigning additional staff to passport adjudication, and hiring additional contractors to surge processing capacity across the country,” a spokeswoman said Tuesday in an email.

       The maximum wait time for “routine service” dropped last week to 16 weeks from 18 weeks, she wrote. She declined to comment on the effect of a shutdown on consular services.

       “We cannot speculate at this time about the impact of a possible lapse of appropriations on consular services,” she said in a statement.