As Republicans scramble on Capitol Hill to get a new government funding agreement passed, many federal workers are now in limbo waiting to see how a government shutdown would impact them with the holidays right around the corner.

Based on past government estimates, some 3.5 million federal workers could go without paychecks this holiday season until an agreement is passed — many of them, like airport security officers, will be required to show up to work anyway. Others could be told to stay home and not work.

A payroll schedule for the U.S. General Services Administration shows some federal workers would miss paychecks as soon as Jan. 3 if an agreement is not reached by then.

Federal contractors are not required to work and are also not guaranteed backpay for the duration of the shutdown.

Since fiscal year 1977, there have been 20 funding gaps, lasting from as short as 1 day to 34 full days in length. If the government shuts down Saturday at 12:01 a.m., it would be the 21st shutdown.

Below is a small snapshot of the impact a shutdown could have on millions of Americans across the country.

Impact on the military

Some 1.3 million active-duty service members would still be required to work, but without pay until a funding agreement is passed. Typically, half of the 700,000 Defense Department civilian workforce would be expected to keep working without pay.

Generally, contractors are not required to work and lose paychecks for the duration of the shutdown.

Impact on travel

The shutdown would come just as many Americans are traveling for the holidays — during some of the busiest travel days of the year.

The Transportation Security Administration says about 59,000 of the agency’s more than 62,000 employees are considered essential and would continue working without pay in the event of a shutdown.

TSA says the agency expects to screen 40 million passengers over the holidays through Jan. 2.

“While our personnel are prepared to handle high volumes of travelers and ensure safe travel, please be aware that an extended shutdown could mean longer wait times at airports,” TSA Administrator David Pekoske said on X.

Nearly 13,000 air traffic controllers would be expected to work without pay, and hiring and training new air traffic controllers would cease during a shutdown, according to the latest planning document from the Department of Transportation.

It’s worth noting that air traffic controllers played a part in ending the government shutdown in 2019, when several workers called in sick on the same day, snarling air traffic. Then-President Donald Trump announced an agreement to temporarily reopen the government the same day.

Impact on the border

An estimated 60,971 employees with U.S. Customs and Border Protection and 17,575 employees with Immigration and Customs Enforcement would be asked to work without pay during a shutdown according to the latest planning document from the Department of Homeland Security.

Would shutdown impact Social Security?

Because they are considered “mandatory” spending, payments from Social Security and Medicare will continue to reach mailboxes, although agencies warn services could slow down.

Could holiday mail be impacted?

The U.S. Postal Service also won’t be impacted because it is an independent agency, and relies on its own revenue stream.

Other impacts Americans could see

Given the shutdown would begin on a Saturday, many Americans would not really feel the impacts of a shutdown until Monday.

The National Park Service said they “remain hopeful that a lapse in government funding will not occur,” but if it does, “routine visitor services will be available through Sunday.” They are still reviewing the agency’s contingency plan and working to determine specifics for individual parks.

The Smithsonian Institution, which operates 21 museums and the National Zoo in Washington, D.C., planned an announcement Friday with more guidance on the impact of a shutdown, a spokesperson told ABC News.