Cape Canaveral (Florida): If Discovery gets off the ground on July 4 (Tuesday), it would be the first manned launch by the United States on the nation's birthday.
It would also be the second liftoff of a space shuttle since the 2003 Columbia disaster.
Launch pad technicians on Monday planned to fuel up the system that will power Discovery, hoping to work quickly enough to make a third liftoff attempt on the Fourth of July.
The technicians began their work almost immediately after the launch was scrubbed Sunday afternoon because of stormy weather, the second delay in two days.
The delays cost NASA an estimated $2 million in overtime pay and fuel costs.
Workers also planned to swap out fruit flies that will be part of an experiment on how space affects the immune system.
NASA hopes to extend the 12-day mission by an extra day to squeeze in a space walk that will test new techniques for repairing damage to the shuttle's thermal skin.
By replenishing the shuttle's on-board fuel now, the astronauts will have a better chance of getting the additional day in orbit.
"It's a strong desire to get that 13th day," said John Shannon, deputy manager of the space shuttle program.
The weather forecast for Tuesday also was better than for Sunday or Monday, with a 40 percent chance that storms at launch time would prevent liftoff, said US Air Force 1st Lt Kaleb Nordgren, a shuttle weather forecaster.
The forecast worsened a bit for Wednesday but improved again for Thursday. NASA planned to make launch attempts on Tuesday and Wednesday before taking a break on Thursday.
The launch pad workers had a tight schedule, and stormy weather Monday could push back the launch attempt to further in the week.
"Local weather can threaten the ability to get to Tuesday," said Michael Leinbach, shuttle launch director. "It's a very, very tight plan to get all that work done in two days."
Once in orbit, Discovery's crew will test shuttle-inspection techniques, deliver supplies to the international space station and drop off German astronaut Thomas Reiter for a six-month stay.
The decision to launch was made by NASA Administrator Michael Griffin despite the concerns of two top agency managers who wanted additional repairs to the foam insulation on the external fuel tank.
Columbia was brought down by a chunk of flyaway foam, and a piece broke off Discovery's redesigned tank last July, barely missing the shuttle.