A government shutdown could disrupt travel with longer lines at the airport, national park closures and snarled itineraries. Nonessential federal operations would cease at midnight on Tuesday if Congress fails to pass legislation to keep the government functioning.

Many airport employees, including air traffic controllers and Transportation Security Administration staff, would be forced to work without pay. The government’s continuing efforts to strengthen the air traffic control system would also be stymied, with some much-needed training and hiring put on ice.

Though transportation systems, including air and rail, would continue to operate, a shutdown could bring financial losses of up to $1 billion each week for the country’s travel economy and heighten “uncertainty in our travel system,” said Geoff Freeman, the president and chief executive of the U.S. Travel Association, a trade group that promotes travel to and within the country.

“The longer a shutdown drags on, the more likely we are to see longer T.S.A. lines, flight delays and cancellations, national parks in disrepair and unnecessary delays in modernizing travel infrastructure,” Mr. Freeman said in a statement.