The Trump administration's proposal for creating a Space Force as a new military service drew bipartisan skepticism in the Senate on Thursday, with several lawmakers questioning the need for expanding the military bureaucracy.

Acting Defense Secretary Patrick Shanahan pitched the proposal as vital to maintaining what he called America's "margin of dominance" in space as potential adversaries like Russia and China develop the capability to challenge U.S. use of space.

"Both China and Russia have weaponized space with the intent to hold American capabilities at risk," Shanahan told the Senate Armed Services Committee. "Every member of this committee has access to the classified threat picture, but the bottom line is: the next major conflict may be won or lost in space."

Committee members agreed that the U.S. needs to innovate in space and move more quickly to improve defenses of U.S. satellites and other interests in space. But several members, both Republicans and Democrats, expressed skepticism about a Space Force, which is a high priority of President Donald Trump.

Sen. Angus King, an Independent from Maine, said he thinks the current approach, with the Air Force handling the bulk of space responsibilities, is working well.

"I'm genuinely undecided, although as you can tell, I'm skeptical," King said. "I don't think it's broken," he added, referring to the current Pentagon approach to space. "You're doing a good job. Why are we going to 'fix' it?"

Sen. Joni Ernst, an Iowa Republican, also raised doubts.

"I guess we need some convincing that there is a necessity for a sixth branch without our armed forces," she said.

The sharpest criticism came from Sen. Elizabeth Warren of Massachusetts, who is a candidate for the 2020 Democratic presidential nomination.

"None of the ideas I've heard today clearly spell out how a Space Force leads to improved security in space," she said. "Instead, all I see is how a new Space Force will create one more organization to ask Congress for money. And there is no reason to believe that adding an entirely new Space Force bureaucracy and pouring buckets more money into it is going to reduce our overall vulnerability in space. I think the taxpayers deserve better than this."

Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, acknowledged that the administration's proposal leaves some issues unresolved, but he argued that it should be approved to address urgent problems. He called it an "80% solution" that can be refined over time.

Some committee members noted that Air Force Secretary Heather Wilson, who testified alongside Shanahan, had publicly questioned the need for a Space Force in 2017.

Sen. Doug Jones, an Alabama Democrat, asked Wilson whether she would be recommending the creating of a Space Force if Trump had not ordered it. She did not answer yes or no but said Trump has helpfully elevated public discussion of space issues.

"We need to give him credit for that," she said.

A Space Force, if approved by Congress, would be the first new military service since the Air Force was created in 1947. It would be the smallest service by far, with between 15,000 and 20,000 members.

Less controversial is a Pentagon plan to resurrect U.S. Space Command, which existed for many years before being eliminated in 2002. Space Command would be responsible for military operations in space, such as defending satellites, with personnel trained and equipped by the Space Force.