WASHINGTON >> Thousands of intelligence agency employees would be furloughed even as tension over North Korea’s nuclear program remains high. Important biomedical and public health research would be interrupted and possibly damaged. Military veterans would watch helplessly as the processing of their disability claims came to a halt.

Although the government won’t actually close if Congress can’t pass a spending bill by Friday at midnight, there’s plenty that won’t get done should hundreds of thousands of federal employees be barred from working until dysfunctional Washington agrees on a plan.

J. David Cox, national president of the American Federation of Government Employees, said shutdowns can have dramatic impacts as jobs are left unattended. The longer a shutdown lasts, he said, the worse it gets.

“Day one, the world doesn’t fall apart,” Cox said. But “things start to crumble” over time, he said, as Americans begin to realize how reliant they are on the government.

Partial shutdowns can be expensive, too. Five years ago, when swaths of the federal government were shuttered for just over two weeks, 850,000 employees were furloughed, which cost the government 6.6 million days of work and more than $2.5 billion in lost productivity and pay and benefits for employees.

In the case of a shutdown, just under half of the 2 million civilian federal workers would be forced off the job if the Trump administration sticks to the rules followed by previous Presidents Ronald Reagan, George H.W. Bush, Bill Clinton and Barack Obama. But U.S. troops will stay at their posts and mail will get delivered as about 500,000 Postal Service employees and 1.3 million uniformed military personnel are exempt from being furloughed.

A lengthy shutdown could cause lingering problems for the Internal Revenue Service, which is preparing for the start of the tax filing season while also still ingesting the sweeping changes made by the new GOP tax law.

It’s “too early to tell” whether a shutdown would affect the government’s implementation of the massive new tax law enacted last month, White House Budget Director Mick Mulvaney said in an interview Wednesday on Fox Business Network’s “Varney & Co.” He said his agency hasn’t yet determined whether the IRS employees developing new tax forms to conform with the law are deemed essential, meaning they would still report to work during a shutdown.

But Marcus Owens, who for 10 years headed the IRS division dealing with charities and political organizations, said those employees traditionally are considered non-essential. In fact, “Nobody in the (IRS) Washington office is going to be at work if there’s a shutdown,” he said.

The Republican architects of the tax law have promised that millions of working Americans will see heftier paychecks next month, with less money withheld by employers in anticipation of lower income taxes. The IRS recently issued new withholding tables for employers.

“I would say it’s a virtual certainty” that the larger paychecks will be delayed if there’s a lengthy shutdown, Owens said.

At the State Department, spokeswoman Heather Nauert said that Secretary of State Rex Tillerson and individual U.S. embassies have some discretion over how to handle a shutdown but that the department was taking direction from the White House’s Office of Management and Budget.

“We will be prepared for all contingencies, including of a lapse” in funding, Nauert said.

She said that while security for American diplomats overseas wouldn’t be affected, no decisions had yet been made about what services, like visa processing and passports, the State Department would be able to provide during a shutdown. Nor has there been a decision about whether Tillerson can go ahead with a planned trip to Europe next week if the government shuts down, she said.

“We’re not going to make any decisions until we need to,” Nauert said about the trip.

The workforce at the 17 U.S. intelligence agencies would be pared way down, according to a person familiar with contingency procedures.

The official, who was not authorized to publicly discuss the matter and spoke on condition of anonymity, said employees that are considered essential and have to work will do so with no expectation of a regular paycheck.

While they can be kept on the job, federal workers can’t get paid for days worked during a shutdown. In the past, however, they have been repaid retroactively even if they were ordered to stay home.

Interior Department spokeswoman Heather Swift said national parks and other public lands “will remain as accessible as possible while still following all applicable laws and procedures.” But she said services at parks that require staffing and maintenance, such as campgrounds, full service restrooms, and concessions won’t be operating if there’s a shutdown.

The Federal Aviation Administration represents the majority — 45,000 — of the Department of Transportation’s more than 58,000 employees. FAA employees in “safety critical” positions would continue to work, including air traffic control and most aviation and railroad safety inspectors.

But certification of new aircraft, processing of airport construction grants, registration of planes and issuance of new pilot licenses and medical certificates would stop.

The Federal Highway Administration and the Federal Motor Carrier Safety Administration, whose operations are mostly paid for out of the Federal Highway Trust Fund, would continue most of their functions. The fund’s revenue comes from federal gas and diesel taxes, which would continue to be collected. But work on issuing new regulations would stop throughout the department and its nine agencies. Federal contractors with money still in the pipeline would also continue to work as long as they don’t require access to federal facilities.

A government shutdown would be disruptive to research and morale at the National Institutes of Health but would not adversely affect patients already in medical studies, according to Dr. Anthony Fauci, the agency’s infectious disease chief.

“We still take care of them,” he said of current NIH patients. But other types of research would be seriously harmed, Fauci said Thursday.

“It’s a scramble to address the possibility,” Fauci said.

A shutdown could mean interrupting research that’s been going on for years, Fauci said. The NIH is the government’s primary agency responsible for biomedical and public health research across 27 institutes and centers. Its research ranges from cancer studies to the testing and creation of vaccines.

“You can’t push the pause button on an experiment,” he said.

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Associated Press writers Joan Lowy, Andrew Taylor, Laurie Kellman, Deb Riechmann, Matthew Lee and Marcy Gordon contributed to this report.

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Contact Richard Lardner on Twitter at http://twitter.com/rplardner