SpaceX called off the flight over a last-minute technical issue with the rocket's launch pad, officials said on a livestream of the launch countdown.
It was not immediately clear when the next launch opportunity would be. The reason for the scrub suggests that SpaceX and NASA could try to launch again in the coming days.
NASA had been set to launch a SpaceX rocket from Florida carrying a replacement crew for the International Space Station in a mission that would set up the return to Earth of Wilmore and Williams - stuck in space for nine months after a trip on Boeing's faulty Starliner.
The US space agency had moved up the mission by two weeks after President Donald Trump and his adviser Elon Musk, CEO of SpaceX, called for Wilmore and Williams to be brought back earlier than NASA had planned.
A planned eight-day stay on the orbiting station has dragged on for Wilmore and Williams, a pair of veteran astronauts and US Navy test pilots. Starliner returned to Earth without them last year.
SpaceX's rocket had been scheduled to blast off from the Kennedy Space Center in Cape Canaveral with a crew of two US astronauts and one astronaut each from Japan and Russia.
Wilmore and Williams have been working on research and maintenance with the space station's other astronauts and have remained safe, according to NASA. Williams told reporters in a March 4 call that she is looking forward to seeing her family and pet dogs upon returning home.
"It's been a roller coaster for them, probably a little bit more so than for us," Williams said of her family.Â
"We're here, we have a mission - we're just doing what we do every day, and every day is interesting because we're up in space and it's a lot of fun."
The flight, known as Crew-10, normally would be considered a routine astronaut rotation. Instead, it has become entangled in politics as Trump and Musk have sought - without offering evidence - to blame former president Joe Biden for the delayed return of Wilmore and Williams.
The demands by Trump and Musk for an earlier return were an unusual intervention in NASA's human spaceflight operations. The mission previously had a target date of March 26, but NASA swapped a delayed SpaceX capsule with a different one that would be ready sooner.
When the new crew arrives aboard the station, Wilmore and Williams and two others - NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov - can return to Earth in a capsule that has been attached to the station since September, as part of the prior Crew-9 mission.
