A government shutdown could snarl plans for tens of millions of Americans traveling for the December holidays if Congress fails on Friday to pass legislation to keep the government functioning. Nonessential federal operations would cease at 12:01 a.m. on Saturday.

For the busy holiday travel period, the Transportation Security Administration expects to screen nearly 40 million air passengers between Thursday, Dec. 19, and Thursday, Jan. 2, a 6.2-percent increase over the same period last year.

The vast majority of essential T.S.A. employees would continue to work, but without pay, said R. Carter Langston, press secretary for the agency, on Thursday. But if some agents, citing financial hardship, stop showing up for work, as was the case during the 2018-19 shutdown, travelers at airports could face longer lines.

“Triggering a damaging government shutdown would hurt families who are gathering to meet with their loved ones,” said Karine Jean-Pierre, the White House press secretary, in a statement issued on Wednesday.

According to the U.S. Travel Association, a trade group, a shutdown could bring financial losses to domestic travel operators of as much as $1 billion per week.

Weather could worsen any shutdown-related delays with storms in the Northeast forecast for the weekend and storms possible on Christmas Eve and Christmas Day in the South.

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