The debate over setting up a new military space branch under the umbrella of the Air Force is heating up on Capitol Hill, and the House could vote this week on whether to launch the proposal.
Ahead of the possible vote, the White House said creating a space corps was "premature at this time."
Mattis, in a letter to Rep. Mike Turner -- an Ohio Republican leading the congressional effort against the Space Corps -- said he was opposed to adding "additional organizational and administrative tail" to the Pentagon.
"At a time when we are trying to integrate the Department's joint warfighting functions, I do not wish to add a separate service that would likely present a narrower and even parochial approach to space operations," Mattis wrote.
The idea for a Space Corps was proposed last month by Alabama Rep. Mike Rogers, chairman of the House armed services strategic forces subcommittee.
Rogers argues it's necessary for the US military to have a dedicated space force to stay ahead of rivals Russia and China, because the Air Force has made space a second priority behind more traditional air needs.
"They use space as a pay for," Rogers told CNN. "And if we segregate the space professionals away from the air dominance professionals, that money pot goes with the space professionals. That's what this all boils down to."
Rogers' proposal was included in the armed services committee's version of the National Defense Authorization Act, a massive $696 billion defense policy bill that's on the House floor this week. He had the support of armed services chairman Texas Rep. Mac Thornberry and the panel's top Democrat, Rep. Adam Smith of Washington state.
Air Force leaders immediately came out against the idea, expressing concerns about a new military branch within the Air Force in the same vein that the Marine Corps is within the Department of the Navy.
But the opposition from the White House and Mattis specifically raises the profile of the administration's push to stop it.
Turner, another senior Republican on the panel, is leading the House effort to scuttle the creation of a Space Corps, and he's proposed an amendment to the defense policy bill to study the issue first before moving forward with it.
"Restructuring the bureaucracy to the grave extent of creating another service branch is extreme," Turner told reporters. "For the House itself and for the enormity of this task, there's a lot more work that needs to be done for us as a body to deliberate and undertake this."
The Space Corps debate makes for a rare instance in the House where the dispute cuts across party lines, and a vote on the issue is likely to make for strange bedfellows in the House.
The House rules committee is considering Wednesday whether to allow a vote on Turner's amendment. It's one of more than 400 amendments that have been filed to the NDAA.
Turner has launched a whip operation to drum up opposition to the measure, getting House appropriations committee Chairman Rodney Frelinghuysen and House intelligence committee chairman Devin Nunes on the record opposing his amendment.
Turner also went to a meeting of the House Freedom Caucus on Tuesday to pitch his opposition to the Space Corps.
Mattis' letter weighing in on a specific amendment is also a rare step for a defense secretary to take while a bill is on the floor, which Mattis acknowledged in his letter.
"I do not routinely comment on potential floor amendments to pending legislation; however, this particular issue warrants a response," Mattis wrote.
At the same time, if the House votes in favor of establishing a Space Corps, it makes the idea more likely to become a reality. That's because a vote by the full House will strengthen the chamber's hand in conference committee negotiations with the Senate, which had no similar proposal in its version of the defense policy bill.