The announcement that United Airlines has chosen Dulles International Airport for a hub of its East Coast traffic is expected to increase parking shortages, baggage delays and traffic congestion that recently have arrived at what had been the region's most underused airport.

"The facilities are becoming overwhelmed," said Clark Onstad, a vice president of Texas Air Corp., which owns New York Air and Continental Airlines. "They're in a heck of a bind out there, and I'm not talking about terminals. I'm talking about the basics."

United and airport executives said yesterday, however, they are confident that their expansion plans will be adequate. But they acknowledged that the plans are based on a gamble: that Congress will pass legislation transferring Dulles and National airports from the Federal Aviation Administration to a regional authority.

Such an authority would be able to float revenue bonds needed to finance many of the more than $240 million in what officials say are critically needed improvements at the two airports. The federal government has been reluctant to make the improvements because of the federal deficit.

The transfer bill is awaiting action by the full Senate, and supporters hope to get a Senate vote next week. Maryland officials are strongly opposed to the measure, and Sen. Paul S. Sarbanes (D-Md.) is expected to try to prevent a vote before the end of the year.

Even before the United announcement, Dulles supporters argued that passage of the transfer legislation was crucial to the health of the airport, which Presidential Airways and New York Air recently began using as a hub. Federal and Virginia officials said that the United decision has significantly raised the stakes.

Martha V. Pennino, vice chairman of the Fairfax County Board of Supervisors, said United's move signals that Dulles "has come of age" and increases the need for better roads and other improvements around the airport.

"It's becoming increasingly difficult, and increasingly we're relying on temporary and interim arrangements" that are designed "to buy time" until the transfer bill is considered, said James A. Wilding, the FAA's director of metropolitan Washington airports. "This argues more and more for the transfer to take place as quickly as possible."

"This will cause the airport to grow more dramatically than it already has," said Thomas G. Moor, president of the Washington Dulles Task Force, a group that has supported better use of the airport. "There's no question that this kind of growth all at one time is going to create strains on the terminal facilities."

At Dulles yesterday, it was easy to see why many are alarmed. By midafternoon, a few hours before the usual weekend crunch, the main parking lot in front of the terminal was nearly filled. Cars, taxis and airport shuttle buses jammed the access road, creating tie-ups that left frustrated motorists leaning on their horns and passengers racing -- in many cases with baggage flung over their shoulders -- toward the terminal.

"We need a bigger airport for everything that's coming in," said Barneyo Grayson, an airport baggage man. He said that parking is a serious problem and that the ramp leading from the lot to the terminal is too small to handle the numbers of passengers.

Onstad said that Dulles "will lose out" to airports such as those in Raleigh-Durham, N.C., and Nashville, which also are seeking to become preeminent airports on the East Coast, unless "it is able to support the kind of growth that these airlines are willing to give it."

Wilding said that Dulles has plans to deal with these problems, but all revolve around the transfer bill. Asked what Dulles officials will do if it is not enacted, Wilding replied: "We really don't think in those terms. We really just got all of our marbles on that one."

Maryland officials have argued that the infusion of cash at Dulles would give it a competitive edge over the state-owned Baltimore-Washington International Airport. Yesterday, a top Maryland transportation official said the recent surge at Dulles supports the state's contention that the proposed sale price of $47 million for the two airports should be higher.

Noting that Presidential and New York Air have built temporary facilities at Dulles on their own, and that United is planning a temporary terminal, David Chapin, director of policy for the Maryland agency, questioned why the airlines could not build permanent facilites there.

Dulles and National are the only commercial airports in the country owned by the federal government. Most airports are operated by authorities, which issue bonds to pay for improvements. The authority then recoups the money from landing fees and other charges.

Until recently, Dulles was a seriously underutilized airport, scorned by many as a white elephant. But with the extension of Rte. I-66 and a direct highway link into Washington, the airport has boomed.

It had 530,000 passengers in October, up 63 percent from the same month of 1984. For the 12 months that ended in October, Dulles handled 4.7 million passengers, an increase of 36.5 percent from the previous 12 months.

"It's really messed up," Kinsley Anduh, a driver for Imperial Cab Co., said yesterday as he surveyed the chaotic scene outside the terminal.

Inside, Stacy Douglass of Charlottesville was waiting in a line to buy tickets to Portland, Maine. "They told me there would be no guarantees, and now I see what they mean," she sighed.

"I've been in quite a few airports, and so far this is the longest I've had to wait," Douglass said. "If things are like this every day, this airport should be bigger."