WASHINGTON - US President Donald Trump's administration on Thursday announced an ambitious plan to usher in a new "Space Force" as the sixth branch of the military by 2020, but the proposal was scorned by opponents and may struggle to get liftoff in a divided Congress.

Trump has strongly championed the idea of creating a space-focused military service with the same stature as the Air Force and the Army, turning his dreams of a "Space Force" into a rallying cry for supporters at political events.

His 2020 reelection campaign sent a fundraising email on Thursday asking supporters to vote on their favorite Space Force logo for future Trump campaign merchandise, offering a choice of six.

US Vice-President Mike Pence, in an address at the Pentagon, described the Space Force as "an idea whose time has come".

"America will always seek peace in space, as on the Earth. But history proves that peace only comes through strength, and in the realm of outer space, the United States Space Force will be that strength in the years ahead," Pence said. He added that Congress must now act to establish and fund the department.

Trump tweeted: "Space Force all the way!"

The Space Force would be responsible for a range of crucial space-based US military capabilities, which include everything from satellites enabling the Global Positioning System (GPS) to sensors that help track missile launches.

But critics view its creation as an unnecessary and expensive bureaucratic endeavor, a vanity project that simply strips away work already being done effectively by services like the Air Force.

Democratic Senator Brian Schatz, who is on the Defense Appropriations subcommittee, said the Space Force was a "dumb idea."

"Although 'Space Force' won't happen, it's dangerous to have a leader who cannot be talked out of crazy ideas," Schatz said on Twitter.