Three years behind schedule and almost $360 million above budget, the Capitol Visitor Center prepares to open its doors to millions of tourists who now must endure long lines without food, restrooms or shelter to catch a glimpse of the halls of Congress.

The underground center, the largest single construction project in the Capitol's two-century history in terms of size and expense, is to open to the public on Dec. 2. The final cost of the project is put at $621 million, more than double the $265 million estimated cost had the center been completed on schedule in December, 2005.

For the 3 million tourists who visit the Capitol every year, the facility is long overdue. People now form lines at the bottom of Capitol Hill and wait in the heat, the cold and the rain to sign up for tours. They then must trek up the hill to enter the building.

With the Capitol Visitor Center, located below ground between the Capitol and the Supreme Court, visits will begin in the vast Emancipation Hall filled with statues moved from the Capitol and a model of the Statue of Freedom that is perched above the Rotunda. The actual Capitol Dome looms overhead through skylights. Dec. 2 is the 145th anniversary of the raising of the statue atop the Dome.

Before beginning tours of the Capitol itself, people can stroll through an exhibition hall with historic documents, artifacts and interactive computers, see shows in two theaters and eat at a 530-seat restaurant area. There are two gift shops and 26 public restrooms, compared to five inside the Capitol.

Among the artifacts are a letter from George Washington to the Continental Congress reporting the defeat of the British at Yorktown, Franklin Roosevelt's "Day of Infamy" speech and John Kennedy's message to Congress proposing travel to the moon.

Also on view is the catafalque, the raised bier first built to support the casket of Abraham Lincoln and since used when presidents and military leaders such as JFK, Douglas MacArthur and Reagan have lain in state in the Rotunda.

Visitors will also be able to book tours of the Capitol in advance at www.visitthecapitol.gov, a Web site to open on Nov. 14, or by phone, 202-226-8000.

The idea of the visitor center dates back to the 1970s, and in 1991 Congress authorized funds for planning.

But momentum for the project did not come until 1998, when a mentally unstable man burst through the doors of the Capitol, killing two police officers before being subdued in the office of then-Republican Whip Tom DeLay. That impressed on lawmakers the need to move security stations for visitors away from the main building. The groundbreaking ceremony took place in 2000.

Security was also a key factor in the cost overruns. After the Sept. 11 attacks, Congress decided to add two tunnels, one for truck deliveries and one linking the Capitol with the Library of Congress, that could also serve as emergency evacuation routes.

Despite grumbling from some lawmakers about rising costs, Congress also approved the addition of House and Senate office space. Then there were the usual overruns associated with a project where 9,000 workers set more than 400,000 pieces of stone, some weighing as much as 500 pounds. The excavation phase required the removal of 65,000 truckloads of dirt.

Acting Architect of the Capitol Stephen Ayers, at a news conference Monday, defended the center, "a treasure in itself," that would both enhance security and contribute to the experience of visiting the Capitol. "I don't think it's extravagant," he said. "We have built a building that's here to last another 215 years."

Changes were still taking place even as the opening approached. Sen. Jim DeMint, a South Carolina Republican, balked at the budget because he said the center's exhibits ignored the nation's religious heritage.

To avoid further delays, Senate Rules Committee Chairman Dianne Feinstein, D-Calif., and top Republican Robert Bennett of Utah agreed in principle to several changes, including engraving "In God We Trust" in stone in a prominent place. The cost: an additional $150,000.