WASHINGTON — Colorado Republican Rep. Mike Coffman had already decided to lead opposition in the U.S. House to President Donald Trump’s “Space Force” proposal.

But a new, widely leaked Air Force estimate that creating a space force as a new military service would cost $13 billion over the first five years only stiffened Coffman’s resolve. Coffman, who chairs the House Armed Services Committee’s Military Personnel Subcommittee and sits on its Strategic Forces Subcommittee, was sure other lawmakers agree with him.

“A really bad idea is a ‘Department of Space,’” Coffman said in an interview Tuesday, adding, “I feel confident we can block this. The president will not have the votes.”

Creating a “separate but equal” Space Force as Trump has proposed will require congressional action. The administration is expected early next year to submit proposed legislation, which for now let the Air Force’s estimate take center stage.

To Coffman, whose district includes Buckley Air Force Base and the 460th Space Wing, the Air Force has not focused enough on space, while China and Russia work to threaten U.S. military’s space-based assets.

It’s vital that the U.S. has assured access to space and that its assets be defensible, but he said he favors the approach in the 2019 National Defense Authorization Act, which establishes a new combatant command for space. He’s not convinced a new service would deliver more capability for the price.

“I think we have made some progress reducing bureaucracy in the Pentagon,” Coffman said of the HASC's recent work. “I just can’t imagine going in the other direction for no real value.”

Susanna Blume, an analyst with the Center for a New American Security, said the cost estimate raises concerns about impacts for the Pentagon, but with so much undecided about what a Space Force will do, it’s hard to speculate who the winners and losers would be.

“You don’t create a large chunk of new bureaucracy for free. And given that there is no new growth in the defense budget on the horizon, yes, things will have to be cut to pay for it,” Blume said. “I think the Air Force is likely to bear the brunt of it, as that’s where the preponderance of the space mission resides now."

Clouding things further, the overall price tag may not be realistic, according to Todd Harrison, a space expert with the Center for Strategic and International Studies.

“This estimate is way too high. It’s as if the Air Force is trying to sabotage the idea by making it seem much broader and more expensive than it really would be,” Harrison said, who added that part of the $13 billion is actually earmarked for U.S. Space Command.

With the $13 billion cost estimate, opponents on Capitol Hill can make it an argument against Pentagon waste. To boot, Coffman said he can point to the initial opposition from Defense Secretary Jim Mattis and Air Force Secretary Heather Wilson, who asked for a chance to address the problem better with the existing organization. (Both have since fallen behind the president.)

Though HASC Strategic Forces Subcommittee Chair Mike Rogers, R-Ala., is a vocal proponent of Space Force, a split among Republicans could be deadly to the proposal. That’s especially so because HASC ranking Democrat Adam Smith, of Washington state, in recent days came out as a strong opponent.

Smith could galvanize Democrats, just as they have a strong chance of taking control of the House in midterm elections. In the scenario, Smith would likely become chairman of the House Armed Services Committee.

Meanwhile, key Republicans in the Senate seemed open-minded but lukewarm on Tuesday.

Senate Appropriations Committee Chairman Richard Shelby, R-Ala., had yet to see the Air Force estimate. He did have a lot of questions and called the $13 billion figure, "a lot of money."

"We'll evaluate it all. We have to look at the cost, purpose, what would it do, will this add another layer of bureaucracy, will it add another layer of security," Shelby said. "What does this do to the Air Force, does it strengthen it, weaken it, create another agency? They've got some explaining to do, I believe."

Shelby was deferential to the Senate Armed Services Committee which has yet to hold a single hearing on this presidential priority. On Tuesday, Senate Armed Services Committee Chairman Jim Inhofe, R-Okla., would not commit to holding hearings on the idea.

Inhofe was initially skeptical of a sixth branch and has yet to take a strong position since Trump threw his weight behind the idea. Last month, he said he was waiting to hear the cost after Mattis appealed to him in a recent meeting.

"The Air Force is primarily affected by this. I think they've been doing a pretty good job, but I don't know what you get for $13 billion, so I don't know. I haven't changed [positions]," Inhofe said.

Inhofe acknowledged the tension between the potential price tag and statutory budget caps, set to return for 2020 and 2021: "You're adding $13 billion over a period of time we may determine could be better spent on something else."

Virginia Democratic Sen. Tim Kaine, a member of the Senate’s Armed Services Committee and its Budget Committee, said it was hard to take Trump’s proposal seriously yet.

“I’m just struck by the fact that the president and others have talked about this and have not come to the Armed Services Committee to share what it means, how they would do it and what it would cost,” Kaine said. “That leads me to question the seriousness of the proposal.”