President Donald Trump's proposed Space Force would be the first new branch of the U.S. military in more than 70 years — if Congress approves and if it can get off the ground.

Vice President Mike Pence outlined the potential new service on Thursday in a speech at the Pentagon, saying the administration hopes to have it up and running by 2020.

It's not the first time the idea has come up. In January 2001, shortly before President George W. Bush took office, a commission led by his soon-to-be defense secretary, Donald Rumsfeld, recommended "a military department for space" (PDF). But the proposal, known as the Rumsfeld Report, was largely forgotten after the terrorist attacks of Sept. 11.

This time, Pence said, the Space Force will be a full branch of the military equal to the Army, the Navy, the Air Force, the Marine Corps and the Coast Guard. The Defense Department said in a report released Thursday that it would be led by a four-star general or an equivalent flag officer.

Trump's 2020 re-election campaign jumped fully in line on Thursday, sending out a letter to supporters offering Space Force-branded apparel for sale "to celebrate President Trump's huge announcement" and asking them to vote on six proposed Space Force logos.

Trump campaign asks supporters to vote for the new Space Force logo pic.twitter.com/rIvd8AcHEh — Kasie Hunt (@kasie) August 9, 2018

Pence pointed to Russia and China, whose space programs are closely tied to their militaries, as well as North Korea and Iran, as threats that must be confronted in space.

In February, China announced that it had carried out a "land-based mid-course missile interception test." While it said the missile that was tested was "defensive in nature," most analysts agreed that it was likely the SC-19, which China used to destroy a satellite in orbit in 2007, or the DN-3, which it used to destroy a test missile above the Earth's atmosphere in 2010.

In March, Russia conducted another flight test — the sixth so far — of an anti-satellite missile system called PL-19/Nudol. The U.S. intelligence community contends that the system is designed to shoot U.S. satellites out of orbit.

Both nation's systems are considered "strategic space warfare weapons," according to the nonprofit defense think tank GlobalSecurity.org. Private and government military experts have long warned that systems designed to knock satellites out of orbit threaten to cripple satellite communications and U.S. ships' ability to navigate by GPS.

"As their actions make clear, our adversaries have transformed space into a war-fighting domain already, and the United States will not shrink from this challenge," Pence said.

But the path to creating a Space Force will be difficult.

The first hurdle will be Congress, which must approve any new military branch, something it hasn't done since 1947, when the Air Force was carved out of the Army's air service.

Congressional budget leaders are already on record opposing the idea — House and Senate negotiators killed it in discussions over the defense budget last year.

And while Trump wants the Space Force in place in less than a year and a half, Deputy Defense Secretary Patrick Shanahan and Air Force Gen. Paul Selva, vice chairman of the Joint Chiefs of Staff, told reporters on Thursday that the Defense Department has yet to work out any specifics.

"We haven't put together a legislative proposal," Shanahan said.

Selva acknowledged that "the first steps are to make sure that you do no harm to the missions that are being accomplished today."

Deborah Lee James, who was secretary of the Air Force during President Barack Obama's second term, said Thursday that she thinks the idea "will have a tough road" on Capitol Hill.

"I think it does have some support in the House of Representatives," James said in an interview on MSNBC, but "I think the Senate will question it very strongly."

Then there's the opposition of many space and military experts, who warn of the red tape that could wrap up another giant military department.

Among them at times has been Trump's own defense secretary, James Mattis, who urged congressional negotiators last year to kill funding for a Space Force, saying in a letter (PDF) to Sen. John McCain, R-Ariz., chairman of the Senate Armed Services Committee, that he was "focused on reducing overhead and integrating joint war-fighting functions."

Mattis, who works directly for Trump, has since come around, at least in public. "We are in complete alignment with the president's concern about protecting our assets in space to contribute to our security to our economy," he said at a news conference in Washington on Tuesday.

But NBC News reported in June that when Trump revived the Space Force idea, he did so without consulting Mattis and over his private objections. Perhaps tellingly, Trump didn't mention Mattis at all in his speech promoting it that day.

"Obviously, General Mattis wasn't sold on this idea," said retired Navy Capt. Mark Kelly, a former astronaut who began piloting space shuttle missions in 2001.

"I don't think he still is today, and others within [the Pentagon], as well," Kelly said Thursday in an interview on MSNBC. "I haven't heard of any big fans out there for this."

Some of the resistance stems from the fact that the United States already has a Space Command. It's part of the Air Force, and it reports to both the secretary of the Air Force and to U.S. Strategic Command, or STRATCOM, the military's joint structure overseeing the U.S. nuclear arsenal.

"You know, the Air Force Space Command already does this. It's at Peterson Air Force base in Colorado," Kelly said.

"They do this with about 20,000 people," he said, adding: "It doesn't make sense to build a whole other level of bureaucracy."

The proposal outlined Thursday doesn't say what would happen to Air Force Space Command, but the Defense Department's report suggests that it would be combined with other government military space programs, initially under the leadership of the head of the current Air Force command.

That would make the Space Force, at least at first, a central repository of mostly existing government initiatives. And that off-the-shelf approach is reflected in Trump's plan to ask Congress to appropriate $8 billion over five years to establish it.

That's less money, spread over five years, than the $12.5 billion that the Defense Department requested for military space programs in the budget simply for fiscal year 2019 (PDF), which Trump signed in March. And it's less than half of the $20 billion proposed next year for NASA, the civilian space agency.

"The question is: 'Will it be workable, will it be a smooth, well-oiled machine?'" James said. "I think the answer is no."

James said a separate Space Force would be counterproductive, because "military services do not do the war-fighting in this country."

"Those are done by combatant commands and unified commands," which span all of the services, James said. "They are the ones who do strategy [and] concepts of operation."

Adding another full military branch that those commands would have to navigate would "sap time, attention and focus away from the real issue, which is we need to invest more in technology and capability in space," she said.

Kelly said: "The only person that I've heard say this is a fantastic idea is the commander in chief, the president of the United States. Everybody else says it's redundant, it's wasteful. We don't have the need out there right now."