* Launch delayed at least until Monday
* Obama meets shuttle crew, astronaut wife Giffords
* Heaters on power unit failed four hours before launch
(Updates with Obama met Giffords)
By Irene Klotz and Alister Bull
CAPE CANAVERAL, Fla., April 29 (Reuters) - NASA delayed Friday's launch of space shuttle Endeavour because of a glitch but the grounded crew and the commander's wounded wife got a chance to greet President Barack Obama as he toured the Kennedy Space Center.
Endeavour's launch was scrubbed four hours before liftoff after the heaters failed on one of the ship's power units. NASA said it would be at least Monday before it could try again to launch Endeavour on its final voyage.
U.S. Representative Gabrielle Giffords, who is married to Endeavour commander Mark Kelly, was on hand for the launch. The Arizona Democrat was gravely injured during an assassination attempt on Jan. 8 and had not been seen publicly since then.
Obama met privately with Giffords before greeting the six crewmen, the White House said.
"I bet you were hoping to see a rocket launch today," Kelly told the president.
Endeavour had been filled with 500,000 gallons (1.9 million litres) of supercold liquid oxygen and liquid hydrogen in preparation for a launch attempt at 3:47 p.m. EDT (1947 GMT). The astronauts were on the way to the launch pad when the flight was called off.
"Unfortunate for the (Endeavour) team and Mark Kelly and his crew, but today the orbiter is not ready to fly and as we always say in this business, we will not fly before we're ready," said launch director Mike Leinbach.
The failed heaters are on one of the shuttle's three onboard generators that provide hydraulic power for the steering systems.
Leinbach said it would be at least 72 hours before there could be another launch attempt.
Endeavour is to deliver a high-profile physics experiment to the International Space Station. The mission is the 134th and next to last for the U.S. space shuttle program, which is ending after a cargo run by the shuttle Atlantis this summer.
Endeavour was the replacement ship for Challenger, which was lost in a fatal 1986 launch accident. It is the youngest of NASA's three surviving spaceships and the second to be retired.
Discovery, the fleet leader, returned from its final space mission in March, and Atlantis is due to end the 30-year-old shuttle program with a launch on June 28.
Obama, who had been scheduled to watch the launch with his wife and daughters, got a close-up look at Atlantis inside a hangar where it is being readied for launch. They stood under a wing as NASA workers explained how the ship's thermal tiles keep it from burning up when it re-enters the atmosphere.
"Where is the duct tape?" Obama joked.
The shuttle Columbia was destroyed due to a damaged heat shield as it flew through the atmosphere for landing in 2003.
Endeavour's last flight is devoted to outfitting the space station, a $100 billion project of 16 nations, with its highest-profile science instrument, a $2 billion particle detector called the Alpha Magnetic Spectrometer.
The instrument was built by an international collaboration of 60 research organizations and overseen by Nobel physics laureate Samuel Ting of the Massachusetts Institute of Technology.
The device is designed to study high-energy particles in cosmic rays, gathering information that could help scientists better understand "dark matter," which cannot be directly detected.
Endeavour also carries spare parts for the station, which will soon be dependent on smaller cargo ships from Russia, Europe and Japan for resupply missions. NASA has hired two U.S. companies, Space Exploration Technologies and Orbital Sciences Corp ORB.N, to join the cargo runs beginning next year. (Editing by Jane Sutton and Eric Walsh)