Nasa hopes to try again to send space shuttle Endeavour on its final voyage on Monday.
President Barack Obama and his family visited Kennedy Space Centre anyway and met Giffords, who is recovering from a gunshot wound to the head and has been in Cape Canaveral since Wednesday to attend her husband's launch.
The White House said Obama saw Giffords for about 10 minutes before meeting the shuttle's crew.
Giffords has not been seen publicly since the assassination attempt on 8 January, and left her Houston rehabilitation hospital for the first time to travel to Florida. It was not immediately known whether she would stay for the next attempt, or return to Houston.
She had been expected to watch the liftoff in private – as were the other astronauts' families.
"Bummed about the scrub!! But important to make sure everything on shuttle is working properly," her staff said via Twitter.
Endeavour was fuelled and the six astronauts were heading to the launchpad when the countdown was halted about three and a half hours before the liftoff, at 3.47pm local time. Nasa's silver-coloured astrovan did a U-turn at the launch control centre and returned the crew to quarters.
It would have been the first time in Nasa history that a sitting president and his family witnessed a launch. As a consolation, Obama and his family got an up-close look at Atlantis. It will make the last shuttle flight this summer as Nasa winds up the 30-year programme and retires the fleet to museums.
The president and his wife met briefly with Endeavour's crew. Obama told the crew he was hoping to get back to Florida for a shuttle launch. "One more chance, we may be able to get down here," he said.
Launch director Mike Leinbach said the next launch attempt for Endeavour would be Monday at the earliest – and hinted at an even longer delay.
Technicians will have to crawl into the shuttle's engine compartment to track a suspected electrical short circuit in a power distribution box.
As many as 700,000 spectators had been expected to visit the area around the launch site.
Endeavour's upcoming mission to the International Space Station is the last in its 19-year history. It will deliver a $2bn physics experiment.
The shuttle – the youngest in the fleet – was built to replace Challenger, destroyed during liftoff in 1986, and made its maiden voyage in 1992.