Less than four hours before the scheduled 3:47 p.m. liftoff, National Aeronautics and Space Administration officials announced that it would be delayed at least until Monday afternoon while technicians tried to fix the problem, which affects equipment that supplies hydraulic power to move the shuttle’s engines and flaps during ascent and re-entry.
Officials suggested that the launching could be delayed further. “We will not fly this machine until it is ready,” said Michael D. Leinbach, the shuttle launching director, “and today it was not ready to go.”
The delay frustrated huge crowds that had gathered outside the space center to watch the start of the next-to-last mission in the 30-year shuttle program.
Mr. Obama, who had been scheduled to watch the launching with his wife, Michelle, and daughters, was touring tornado damage in Alabama when the postponement was announced. He decided to visit the space center anyway, arriving about two hours later.
At the launching control center, the president and his wife met for about 10 minutes with Gabrielle Giffords, the wounded Arizona Democratic congresswoman and wife of the mission commander, Capt. Mark E. Kelly.
Just before going in to see Ms. Giffords, Mr. Obama greeted Captain Kelly in a corridor. The men shook hands and embraced.
The Obamas also met with the five other members of the shuttle crew. With their daughters, they toured the building where the Atlantis spacecraft is being prepared for the 135th and final shuttle mission, in late June. They walked under the shuttle, getting a close look at its thermal tiles.
The astronauts are under quarantine rules before a launching and ordinarily their contact with others, even family members, is restricted, but NASA officials said flight doctors had cleared the Obamas to meet with the crew.
Ms. Giffords, who suffered a severe head wound in a shooting in January, had arrived on Wednesday to watch the launching. There was no immediate word as to whether she would stay in Florida or return to the Houston rehabilitation hospital where she has been recovering for the past two months.
The astronauts had undergone final medical checks and donned orange flight suits and were on their way to the launching pad in a van when the postponement was announced. The van made a U-turn and parked briefly at the control center before returning to the astronauts’ quarters.
Whenever it begins, the mission, to the International Space Station, is scheduled to last two weeks. The Endeavour crew will install a $2 billion particle-physics experiment, the Alpha Magnetic Spectrometer, at the space station, where it will search for evidence of the elusive “dark matter” that is thought to be ubiquitous in the universe. The shuttle will also deliver spare parts, and the crew will perform maintenance tasks outside the station in four spacewalks.
Until Friday, preparations for the mission had been relatively smooth, with weather the major concern. But postponements due to equipment problems are not uncommon, and NASA officials had previously said they would not forgo safety procedures just because the president was going to be there.
Mr. Leinbach said the problem that forced the postponement was most likely an electrical short in an area of the shuttle that would take time to reach. Shortly after the decision was announced, technicians drained the spacecraft’s huge external fuel tank, which had been filled with liquid hydrogen and oxygen fuel early Friday. The fuels must be removed before it is safe to work on the spacecraft.
Mr. Leinbach and other officials said that depending on the extent of the problem, a Monday launching might be possible.
NASA has a launching window each afternoon through Wednesday; after that the Endeavour might have to wait until May 9 or 10 to try again.
Helene Cooper contributed reporting.
