WASHINGTON (AP) — The federal government shutdown only partially curbs operations. But the longer the shutdown continues, the more likely its impact will be felt.

U.S. troops will stay at their posts and mail will get delivered, but almost half of the 2 million civilian federal workers will be barred from doing their jobs if the shutdown extends into Monday.

How key parts of the federal government would be affected by a shutdown:

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INTERNAL REVENUE SERVICE

A shutdown plan posted on the Treasury Department's website shows that nearly 44 percent of the IRS' 80,565 employees will be exempt from being furloughed during a shutdown. That would mean nearly 45,500 IRS employees will be sent home just as the agency is preparing for the start of the tax filing season and ingesting the sweeping changes made by the new GOP tax law.

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.

But Marcus Owens, who for 10 years headed the IRS division dealing with charities and political organizations, said it's a "virtual certainty" that the larger paychecks will be delayed if there's a lengthy government shutdown.

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HEALTH AND HUMAN SERVICES DEPARTMENT

Half of the more than 80,000 employees will be sent home. Key programs will continue to function because their funding has ongoing authorization and doesn't depend on annual approval by Congress. But critical disruptions could occur across the vast jurisdiction of HHS programs — including the seasonal flu program.

Medicare, which insures nearly 59 million seniors and disabled people, will keep going. And so will Medicaid, which covers more than 74 million low-income and disabled people, including most nursing home residents.

States will continue to receive payments for the Children's Health Insurance Program, which covers about 9 million kids. However, long-term funding for the program will run out soon unless Congress acts to renew it.

Deep into a tough flu season, the Centers for Disease Control and Prevention will be unable to support the government's annual seasonal flu program. And CDC's ability to respond to disease outbreaks will be significantly reduced.

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JUSTICE DEPARTMENT

Many of the nearly 115,000 Justice Department employees have national security and public safety responsibilities that allow them to keep working during a shutdown. Special counsel Robert Mueller's team investigating Russian meddling in the presidential election will also continue working. His office is paid for indefinitely.

The more than 95,000 employees who are "exempted" include most of the members of the national security division, U.S. attorneys, and most of the FBI, Drug Enforcement Administration, Bureau of Alcohol, Tobacco, Firearms and Explosives, U.S. Marshals Service and federal prison employees. Criminal cases will continue, but civil cases will be postponed as long as doing so doesn't compromise public safety. Most law enforcement training will be canceled, per the department's contingency plan.

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STATE DEPARTMENT

Many State Department operations will continue in a shutdown. Passport and visa processing, which are largely self-funded by consumer fees, will not shut down. The agency's main headquarters in Washington, in consultation with the nearly 300 embassies, consulates and other diplomatic missions around the world, will draw up lists of nonessential employees who will be furloughed.

Department operations will continue through the weekend and staffers will be instructed to report for work as usual on Monday to find out whether they have been furloughed.

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DEFENSE DEPARTMENT

The U.S. military will continue to fight wars and conduct missions around the world, including in Iraq, Syria and Afghanistan. And members of the military will report to work, though they won't get paid until Congress approves funding.

But Defense Secretary Jim Mattis warned Friday that a shutdown will have far-reaching effects.

"Our maintenance activities will probably pretty much shut down," he said during remarks at the Johns Hopkins School of Advanced International Studies. "We do a lot of intelligence operations around the world, and they cost money. Those, obviously, would stop. And I would just tell you that training for almost our entire reserve force will stop."

And, while ships will remain at sea and airstrikes against enemy fighters will continue, any National Guard forces heading out to do weekend training duty around the country will arrive at armories and be told to go home.

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U.S. INTELLIGENCE AGENCIES

The workforce at the 17 U.S. intelligence agencies will be pared down significantly, 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 who 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 be paid for days worked during a shutdown. In the past, however, they have been paid retroactively even if they were ordered to stay home.

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HOMELAND SECURITY DEPARTMENT

A department spokesman said nearly 90 percent of Homeland Security employees are considered essential and will continue to perform their duties during a government shutdown.

That means most Customs and Border Protection and Transportation Security Administration workers will stay on the job, according to the department's shutdown plan, dated Friday.

Immigration and Customs Enforcement will be staffed at about 78 percent, meaning more than 15,000 of the agency's employees will keep working. The Secret Service, also part of Homeland Security, will retain more than 5,700 employees during the shutdown.

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INTERIOR DEPARTMENT

The Interior Department says national parks and other public lands will remain as accessible as possible. That position is a change from previous shutdowns, when most parks were closed and became high-profile symbols of dysfunction.

Spokeswoman Heather Swift said the American public — especially veterans who come to the nation's capital — should find war memorials and open-air parks available to visitors. Swift said many national parks and wildlife refuges nationwide will also be open with limited access when possible.

She said public roads that already are open are likely to remain open, though services that require staffing and maintenance such as campgrounds, full-service restrooms and concessions won't be operating. Backcountry lands and culturally sensitive sites are likely to be restricted or closed, she said.

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TRANSPORTATION DEPARTMENT

More than half — 34,600 — of the Department of Transportation's 55,100 employees will continue working during a shutdown. The bulk of those staying on the job work for the Federal Aviation Administration, which operates the nation's air traffic control system.

Controllers and aviation, pipeline and railroad safety inspectors are among those who would continue to work.

But certification of new aircraft will be limited, and processing of airport construction grants, training of new controllers, registration of planes, air traffic control modernization research and development, and issuance of new pilot licenses and medical certificates will stop.

At the National Highway Traffic Safety Administration, investigations on auto safety defects will be suspended, incoming information on possible defects from manufacturers and consumers won't be reviewed and compliance testing of vehicles and equipment will be delayed.

The Federal Highway Administration and the Federal Motor Carrier Safety Administration, whose operations are mostly paid for out of the Federal Highway Trust Fund, will continue most of their functions. The fund's revenue comes from federal gas and diesel taxes, which will continue to be collected. But work on issuing new regulations will stop throughout the department and its nine agencies.

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NATIONAL INSTITUTES OF HEALTH

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

"We still take care of them," he said of current NIH patients. But other types of research would be seriously harmed, 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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ENVIRONMENTAL PROTECTION AGENCY

EPA Administrator Scott Pruitt has instructed workers there to come to work next week even with a shutdown. Pruitt said in an email to all EPA employees on Friday that the agency had "sufficient resources to remain open for a limited amount of time." He said further instructions would come if the shutdown lasts for more than a week.

The instructions from Pruitt are different from how the agency has operated during prior shutdowns and the contingency plan posted on EPA's website. A spokesman for the agency said earlier on Friday that the December 2017 plan was no longer valid.

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Associated Press writers Sadie Gurman, Joan Lowy, Michael Biesecker, Lolita Baldor, Andrew Taylor, Ricardo Alonso-Zaldivar, 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