"It will make flying on a normal day seem like you're flying in blizzard weather," Sen. Chuck Schumer, D-N.Y., speaking at New York's LaGuardia Airport, said of the nation's most congested facilities on a normal day.
The furloughs, which began Sunday but didn't cause many delays because of light air travel, are tied to $85 billion in across-the-board cuts known as the sequester.
The Federal Aviation Administration told the nation's airlines last week some 6,700 flights a day, or nearly a third of all U.S. flights, could be delayed at 13 of the busiest U.S. airports.
The delays would come from planes being held on the ground before takeoff or slowed while en route to destinations, the FAA said.
The most-affected airports would likely be Hartsfield-Jackson Atlanta, Chicago O'Hare, Newark Liberty International Airport in New Jersey, Kennedy International and LaGuardia in New York, and Los Angeles International, FAA Administrator Michael Huerta told reporters Friday.
Other likely affected airports were Charlotte (N.C.) Douglas, Chicago Midway, Fort Lauderdale-Hollywood (Fla.), Miami, Philadelphia, San Diego and San Francisco, he said.
Delays at all the big airports likely will ripple to other airports, Huerta said.
The furloughs for the Transportation Department's 55,000 employees -- including 47,000 FAA employees, most of whom are air-traffic controllers -- require a day off without pay for every 10 workdays, or a total of 11 days off, through the end of the fiscal year ending Sept. 30.
The FAA said in a statement Sunday it would "be working with the airlines and using a comprehensive set of air traffic management tools to minimize the delay impacts of lower staffing as we move into the busy summer travel season."
Besides ground stops and ground-delay programs, delays might "also result from other traffic management initiatives, such as increasing spacing between planes," the agency said.
"We cannot allow these furloughs to go through," Schumer said Sunday, recommending Washington instead remove tax breaks for oil companies and raise revenue elsewhere.
"Make some sensible cuts in areas where there's much more waste than here," he told reporters.
The Obama administration has said sequestration, part of the Budget Control Act of 2011, permits no discretion with the budget cuts. The FAA has said the furloughs are unavoidable.
Major U.S. airlines lost a bid in court Friday to halt the furloughs after the U.S. Court of Appeals for the District of Columbia Circuit denied a requested emergency motion to stop them.
The Airlines for America trade group of the carriers and a leading plaintiff was quoted in The Wall Street Journal Sunday as saying it would continue its legal challenge through the judicial and legislative processes to stop "this needless and illegal" plan, which affects shippers as well as passengers.