SpaceX's historic first crewed mission was set to blast off for the International Space Station as scheduled on Saturday, NASA said, although uncertainty remained over weather conditions at the Florida launch site.
"We are moving forward with launch today," NASA chief Jim Bridenstine said in a tweet. "Weather challenges remain with a 50 percent chance of cancellation."
"Proceeding with countdown today," said SpaceX founder Elon Musk, the entrepreneur whose California-based company is seeking to become the first private firm to send astronauts into space.
Weather forced the postponement on Wednesday of what would have been the first launch of American astronauts from US soil since the space shuttle program ended in 2011.
The Falcon 9 rocket with SpaceX's Crew Dragon capsule is scheduled to liftoff at 3:22 pm Eastern Time (1922 GMT) on what is designed to be the final test flight before NASA certifies the spacecraft for regular crew flights.
The next window, which is determined by the relative position of the launch site to the space station, is Sunday at 3:00 pm (1900 GMT), and fair weather is predicted.
NASA astronauts Robert Behnken, 49, and Douglas Hurley, 53, former military test pilots who joined the US space agency in 2000, are to blast off for the ISS from historic Launch Pad 39A on a two-stage SpaceX Falcon 9 rocket.
The same launch pad was used by Neil Armstrong and his Apollo 11 crewmates on their historic 1969 journey to the Moon, as NASA seeks to revive excitement around human space exploration ahead of a planned return to Earth's satellite and then Mars.
The mission comes despite nationwide shutdowns caused by the coronavirus pandemic.
Behnken and Hurley, veterans of two space shuttle missions each, have been in quarantine for more than two weeks and have been regularly tested for COVID-19.
NASA has urged crowds to stay away from Cocoa Beach, the traditional viewing spot -- but that did not deter scores of space fans on Wednesday.
President Donald Trump, who flew in for the previous launch attempt, is expected to attend again.
NASA has had to pay Russia for use of its Soyuz rockets to take US astronauts to space ever since the space shuttle program ended and the decision was taken to shift focus to commercial partners for missions in low Earth orbit.
The Crew Dragon mission, dubbed "Demo-2," is a defining moment for SpaceX, the company founded by Musk in 2002 with the goal of producing a lower-cost alternative to human spaceflight.
SpaceX conducted a successful test flight of Crew Dragon to the ISS in March 2019 with a sensor-laden mannequin on board named Ripley, after the character played by Sigourney Weaver in the "Alien" movies.
By 2012, it had become the first private company to dock a cargo capsule at the ISS, resupplying the station regularly ever since.
Two years later, NASA ordered the next step: to transport its astronauts there by adapting the Dragon capsule.
The US space agency paid more than $3 billion for SpaceX to design, build, test and operate its reusable capsule for six future space round-trips.
The project has experienced delays, explosions, and parachute problems -- but even so, SpaceX has beaten its competitor, aerospace giant Boeing, to the punch.
Crew Dragon is scheduled to dock with the ISS about 19 hours after liftoff, for a duration that is yet to be finalized, but likely around early August.
Wednesday's flight was scrubbed 17 minutes before blastoff because of high levels of atmospheric electricity that could have triggered a lightning strike on the rocket.
With astronauts Behnken and Hurley strapped into the Crew Dragon capsule, the launch pad platform retracted and rocket fueling underway, SpaceX and NASA made the call to abort.
A live video feed showed Behnken and Hurley -- in their futuristic white uniforms adorned with the US flag and the logos of NASA and SpaceX -- waiting as propellant was unloaded from the reusable Falcon 9 rocket after the launch was postponed.