CAPE CANAVERAL, Fla. >> The historic next-to-last space shuttle launch was scratched Friday because of mechanical problems, spoiling a visit from the president and dashing the hopes of the biggest crowd of spectators in years, including the mission commander’s wounded wife, Rep. Gabrielle Giffords.

NASA hopes to try again Monday to launch space shuttle Endeavour on its final voyage.

President Barack Obama and his family visited Kennedy Space Center anyway but it was unclear whether he would still meet with the Arizona congresswoman. Giffords, who is recovering from a gunshot wound to the head, has been in Cape Canaveral since Wednesday to attend her husband’s launch.

Giffords hasn’t been seen publicly since the Jan. 8 assassination attempt, and left her Houston rehabilitation hospital for the first time to travel to Florida. It was not immediately known whether she would stay for another try or return to Houston.

She had been expected to watch the liftoff in private — as were the other astronaut families.

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

Endeavour was fueled and the six astronauts were heading to the launch pad when the countdown was halted, about 3½ hours before the 3:47 p.m. liftoff. NASA’s silver-colored astrovan did a U-turn at the launch control center and returned them to crew quarters.

It would have been the first time in NASA history that a sitting president and his family witnessed a launch; Obama is not planning to return. 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-old program and retires the fleet to museums.

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

"We’ll fly no orbiter before its time, and today she just wasn’t ready to go," Leinbach said.

As many as 700,000 spectators had been expected to crowd nearby coastal communities. For days, police have been warning of massive traffic delays.

Tammi Flythe, among the thousands gathered across the Indian River in Titusville with her two children, was crushed. They traveled in from Tampa, about 130 miles across the state.

"I really wanted my son to experience this," she said.

At the press site at Kennedy Space Center, astronaut James Kelly, a two-time shuttle pilot, took the news in stride. "When you have technical problems, you delay. That’s it. You delay," he said.

"Of course, it’s always disappointing, especially for the crew," added astronaut Clayton Anderson, a former space station resident. "NASA has a great safety record and they’re going to do it the right way. They’re going to pull it back and do what’s right. They have to."

Since arriving in Florida aboard a NASA jet on Wednesday, Giffords’ whereabouts have been kept secret. Her doctors had said she was "medically able" to travel and that they viewed the trip as part of her rehabilitation.

After the launch was called off, a close family friend who was in Florida to see the launch said her family and staff were deciding whether Giffords should stay or return to Houston. The friend, who asked not to be named because he was not authorized to speak for the family, said the decision is not based on her health, but rather logistics.

In late morning, one of the two prime heaters for a fuel line feeding one of Endeavour’s three auxiliary power units failed. At the same time, another heater was acting up.

Leinbach said both main heaters need to be operating for redundancy. The power units provide hydraulic pressure to the main engines at liftoff and to the rudder and speed brake during landing.

The short appears to be in a switchbox or an electrical line leading to it, Leinbach said.

Endeavour’s upcoming mission to the International Space Station is the last in its 19-year history. It will deliver a $2 billion 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.

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AP writers Seth Borenstein and Kyle Hightower in Florida and Ramit Plushnick-Masti in Houston contributed to this report.

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NASA: http://www.nasa.gov/shuttle