WASHINGTON, March 16 (Reuters) - A SpaceX capsule delivered four astronauts to the International Space Station early on Sunday in a NASA crew-swap mission that will allow a pair of stuck astronauts, Butch Wilmore and Suni Williams, to return home after nine months on the orbiting lab.

About 29 hours since launching at 7:03 p.m. ET on Friday from NASA's Kennedy Space Center in Florida, the Crew-10 astronauts' SpaceX Crew Dragon capsule docked to the ISS at 12:04 a.m. ET (0404 GMT) on Sunday.

They were welcomed by the station's seven-member crew, which includes Wilmore and Williams - veteran NASA astronauts and retired Navy test pilots who have remained on the station after problems with Boeing's ( BA ) Starliner capsule forced NASA to bring it back empty.

Otherwise a routine crew rotation flight, the Crew-10 mission is a long-awaited first step to bring Wilmore and Williams back to Earth - part of a plan set by NASA last year that has been given greater urgency by President Donald Trump since he took office in January.

Wilmore and Williams are scheduled to depart the ISS on Wednesday as early as 4 a.m. ET (0800 GMT), along with NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov.

Hague and Gorbunov flew to the ISS in September on a Crew Dragon craft with two empty seats for Wilmore and Williams, and that craft has been attached to the station since.

The Crew-10 crew, scheduled to stay on the station for roughly six months, includes NASA astronauts Anne McClain and Nichole Ayers, Japanese astronaut Takuya Onishi and Russian cosmonaut Kirill Peskov.

The crew-swap mission became entangled in politics as Trump and his adviser Elon Musk, who is also SpaceX's CEO, urged a quicker Crew-10 launch. They claimed, without evidence, that Trump's predecessor Joe Biden had abandoned Wilmore and Williams on the station for political reasons.

Having seen their mission turn into a normal NASA rotation to the ISS, Wilmore and Williams have been doing scientific research and conducting routine maintenance with the other five astronauts.

Williams told reporters this month that she was looking forward to returning home to see her two dogs and family. "It's been a roller coaster for them, probably a little bit more so than for us," she said. (Reporting by Joey Roulette and Shivani Tanna; Editing by William Mallard)