The space shuttle Endeavour sits on launch pad 39A following the scrubbed launch attempt at the Kennedy Space Center in Cape Canaveral, Florida April 29, 2011. [Photo/Agencies]

CAPE CANAVERAL, Florida - A historic space shuttle launch was scratched Friday for mechanical problems, spoiling a visit from the president and dashing hopes of guests like astronaut-wife, congresswoman Gabrielle Giffords and the biggest crowd of spectators in years.

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NASA hopes to try again Monday to launch space shuttle Endeavour on its final voyage, the next-to-last shuttle flight.

President Barack Obama still visited Kennedy Space Center for a tour but it was unclear whether he would meet with the wounded Giffords, wife of the shuttle's commander. She is recovering from a gunshot wound to the head, and it was not immediately known whether she would stay in Florida for another try or return to Houston.

Giffords hasn't been seen publicly since the January 8 assassination attempt. She left her rehabilitation hospital in Houston on Wednesday for the first time to travel to Florida for her husband's launch.

The Arizona congresswoman, who has difficulty walking and talking, was expected to watch the 3:47 pm liftoff in private with other astronauts' families.

"Bummed about the scrub!! But important to make sure everything on shuttle is working properly," her staff said via Twitter.

Launch director Mike Leinbach said the next try would be Monday at the earliest - and hinted at even a longer delay. Technicians will have to crawl into the engine compartment to track a suspected electrical short in a power distribution box, and that will take time.

Leinbach said the delay was "unfortunate" for commander Mark Kelly and his crew. The six astronauts were en route to the launch pad when the countdown was halted. NASA's silver-colored astrovan did a U-turn at the launch control center and returned them to crew quarters.

"It's the nature of our business," Leinbach said. "We'll fly no orbiter before its time, and today she just wasn't ready to go."

The news took guests by surprise as well as journalists who were outside watching the astrovan drive by. There was confusion when the van pulled into the driveway in front of launch control and parked there for several minutes, rather than head straight for the pad three miles away. Then the official announcement came over NASA's broadcast lines.

US President Barack Obama, and first lady, Michelle Obama talk to the astronauts of the space shuttle Endeavour including mission commander Mark Kelly (R) after the mission was postponed at the Kennedy Space Center at Cape Canaveral, Florida, April 29, 2011. [Photo/Agencies]