If Congress does not extend funding beyond Friday, hundreds of thousands of federal employees would be told not to go to work and US government operations would be disrupted.
Employees who are considered indispensable would continue to work. While all federal employees would get retroactive pay once government operations resume, their salary might be interrupted during a shutdown.
Here is a guide to what would stay open and what would shut down, according to agency shutdown plans outlined last year:
Congress writes detailed spending legislation for most US government agencies each year, but it rarely gets the job done before the fiscal year starts on October 1. Lawmakers typically pass stopgap spending bills to avoid disruption for several weeks or months while they finish their work.
The current stopgap bill is due to expire on December 20. Republicans who control the House of Representatives and Democrats who control the Senate have negotiated a 1,500-page bill that extends that funding until March 14, when President-elect Donald Trump will be back in office. But Trump on Wednesday urged lawmakers to vote against it, complicating its chances of becoming law.
If Congress does not pass some sort of extension and Democratic President Joe Biden does not sign it into law before midnight on Friday, wide swaths of the government would not have the money to continue their services in the days leading up to Christmas on December 25.
The US Postal Service would be unaffected because it does not depend on Congress for funding.
What happens to social security, Medicare and other benefits?
The Social Security Administration would continue to issue retirement and disability benefits.
Payments would likewise continue under the Medicare and Medicaid healthcare programmes.
Military veterans' benefits and medical care would also continue.
Food benefits through the Supplemental Nutrition Assistance Program and other nutrition programmes would continue, though they could be disrupted to some extent.
The 2 million US military personnel would remain at their posts, but roughly half of the Pentagon's 800,000 civilian employees would be furloughed — ordered not to work.
Contracts awarded before the shutdown would continue, and the Pentagon could place new orders for supplies or services needed to protect national security. Other new contracts, including renewals or extensions, would not be awarded.
Payments to defence contractors such as Boeing, Lockheed Martin and RTX, formerly known as Raytheon, could be delayed.
The Department of Energy's National Nuclear Security Administration would continue maintaining nuclear weapons.
Agents at the FBI, the Drug Enforcement Administration and other federal law enforcement agencies would remain on the job, and prison staffers would continue to work.
The Secret Service and the Coast Guard would also continue operations, and most employees would continue to work.
Most of the Federal Trade Commission's consumer-protection workers would be furloughed, as would half of its antitrust employees.
National parks, monuments and other sites would be closed to the public, though it will be impossible to keep visitors out of many of them. Rangers would be furloughed and restrooms, help desks and other facilities would be shuttered.
Some states may use their own money to keep parks and other sites open, as happened in the 2018-2019 shutdown.
Wildfire fighting efforts would continue, though timber sales on national forest lands would be curtailed and fewer recreation permits would be issued.
What happens to borders and homeland security?
Most Border Patrol and immigration enforcement agents would continue to work, as would most customs officers.
Local govts would not get new aid to shelter migrants.
The Cybersecurity and Infrastructure Security Agency would suspend security reviews that help schools, local governments and other institutions defend against ransomware.
Federal courts could operate for several weeks without congressional funding, relying on fees and other funds, but eventually would have to scale back activity. The Supreme Court would stay open as well.
Criminal prosecutions would continue. Most civil litigation would be postponed.
Lawmakers would continue to collect paychecks, even as other federal workers do not. Staffers would not get paid, though those deemed essential would be required to work.
Airport security screeners and air-traffic-control workers would be required to work, according to recent contingency plans, though absenteeism could be a problem. Some airports had to suspend operations during a shutdown in 2019 when traffic controllers called in sick.
The Transportation Security Administration would not be able to hire new airport security screeners during the busy holiday travel season.
Some major infrastructure projects could face delays because environmental reviews and permitting would be disrupted, according to the White House.
US embassies and consulates would remain open. Passport and visa processing would continue as long as there were sufficient fees to cover operations. Nonessential official travel, speeches and other events would be curtailed.
Some foreign aid programmes could run out of money as well.
Scientific research would be disrupted as agencies like the National Institutes of Health, the National Science Foundation and the National Oceanographic and Atmospheric Administration would furlough most of their workers once they run out of funding.
National Aeronautics and Space Administration (Nasa) would continue to support the International Space Station and track satellites, but 17,000 of its 18,300 employees would be furloughed.
Weather forecasts and fisheries regulation would continue, as would patent and trademark reviews.
The Federal Communications Commission would suspend consumer-protection activities, equipment reviews and licensing of TV and radio stations. It would continue to distribute telecommunications subsidies and its broadband mapping effort.
The Centers for Disease Control and Prevention would continue to monitor disease outbreaks, though other public health activities could suffer as more than half of the agency's workers would be furloughed.
The National Institutes of Health would furlough most of its staff and delay new clinical trials for medical treatments.
Healthcare services for military veterans and Native Americans would continue.
Most inspections of hazardous waste sites and drinking water and chemical facilities would stop.
Food-safety inspections by the Food and Drug Administration could be delayed. However, the FDA's testing of new drugs and medical devices would continue.
The Securities and Exchange Commission (SEC) would furlough roughly 90% of its 4,600 employees and suspend most activities, it said last year, leaving only a skeleton staff to respond to emergencies.
Likewise, the Commodity Futures Trading Commission would furlough almost all of its employees and cease most oversight activity.
The Federal Reserve, the Federal Deposit Insurance Corporation and the Office of the Comptroller of the Currency would continue as normal because they are funded by industry fees rather than congressional appropriations.
The Financial Industry Regulatory Authority, an industry-financed brokerage oversight body, would continue to operate.
The Treasury Department would continue to issue debt and manage the government's cash position.
The publication of major US economic data, including employment and inflation reports of critical importance to policymakers and investors, would be suspended.
The Internal Revenue Service would stop examining and auditing tax returns and responding to taxpayer queries. Automated tax collection would continue, as would processing of green-energy tax credits under the Inflation Reduction Act. Roughly two-thirds of the agency's 90,000 employees would be furloughed.
Pell Grants and student loans would continue to be paid out, but could be disrupted as most Education Department employees would be furloughed.
A protracted shutdown could "severely curtail" aid to schools, universities and other educational institutions, the department said last year. It also could delay funds that are due to be awarded later in the year.
The White House said last year that 10,000 children from low-income families would lose access to the Head Start preschool program.
The Small Business Administration would not be able to issue any new loans, though loans for businesses hurt by natural disasters would continue.
Meat and egg inspections would continue, but some lab services would be disrupted, making it harder to fight animal diseases. Crop insurance would not be affected, but some loan programmes would be. Research, conservation and rural development programmes would be shut down.
Workplace safety inspections would be limited, and investigations into unfair pay practices would be suspended.
The ability of the National Labor Relations Board to mediate labor disputes would be curtailed because almost all of its 1,200 employees would be furloughed, according to a 2022 plan.
Monthly subsidies for public housing and low-income housing aid would be at risk. The Federal Housing Administration would continue to back insured mortgages, and Ginnie Mae would continue to back the secondary mortgage market. New homebuyers in rural areas would not be able to get loans from the Agriculture Department.
In the 2018-2019 shutdown, the White House furloughed 1,100 of 1,800 staff in the Executive Office of the President. Some offices, such as the National Security Council, continued at full strength, while others like the Office of Management and Budget were scaled back sharply.
The US Constitution specifies that the president continues to get paid.
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