NASA had been set to launch a SpaceX rocket from Florida carrying a replacement crew for the ISS 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 launch was called off due to a hydraulic system issue with a ground support clamp arm for the Falcon 9 rocket, NASA said in a statement on Wednesday.
NASA said the next available launch opportunity is no earlier than 7.26pm on Thursday (local time), pending review of the issue. With a Thursday Crew-10 launch, the Crew-9 mission would depart the space station on March 17, it said.
The US space agency had moved up the mission by two weeks after President Donald Trump and his adviser Elon Musk, the 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 at 7.48pm on Wednesday (local time) 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. "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, 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 last year as part of the prior Crew-9 mission.
Wilmore and Williams cannot leave until the new Crew-10 craft arrives in order to keep the ISS staffed with enough US astronauts for maintenance, according to NASA.
The pair flew to the station in June as the first test crew of Boeing's Starliner, which suffered propulsion system issues in space. NASA deemed it too risky for the astronauts to fly home on the Boeing craft. This led to the current plan to bring them home in a SpaceX capsule.
Boeing built Starliner under a $US4.5 billion ($NZ7.8 billion) contract with NASA to compete with SpaceX's Crew Dragon capsule, which since 2020 has been the US space agency's only vehicle for sending ISS crew members to orbit from American soil.
Last year's mission had marked Starliner's first test flight with astronauts aboard, a requirement before NASA could certify the capsule for routine astronaut missions.
Starliner's development has been plagued with engineering issues and cost overruns since 2019, putting it far behind SpaceX's Crew Dragon, which was developed under a similar NASA contract worth at least $US4 billion.
