Washington (CNN) President Donald Trump's vow to massively rebuild the US armed forces comes with an interesting wrinkle in his first budget request: a call to close military bases.

Trump's Pentagon is asking for a new commission to study closing or revamping bases in its budget request, as the Defense Department says it has upwards of 20% excess infrastructure capacity and slimming down would save money.

But the request to close bases, which would not occur until 2021, is an uphill climb on Capitol Hill. Republicans are already unhappy that Trump did not propose a bigger military budget after his pledge to massively add ships, planes and troops. GOP lawmakers say going through another round of the Base Realignment and Closure (BRAC) process is another step in the wrong direction when the military has been constrained for years by the so-called "sequestration" budget caps.

"The last thing the military needs right now is a BRAC," said Sen. Lindsey Graham, a South Carolina Republican. "But if you can't raise sequestration numbers then you're going to have to do a BRAC."

BRAC was created so the decision of which bases to close is taken out of Congress' hands. The commission gives Congress an all-or-nothing proposal to approve.

Of course, lawmakers have parochial reasons to oppose even studying closing facilities: Closing a base can be devastating for a congressional district's economy due to the loss of service members living in a community as well as civilian jobs. For Trump, it could mean hurting a key constituency -- the military -- and forcing the White House to explain why it calls for more spending but major cutbacks in bases.

Earlier this year, during a visit to the USS Gerald Ford in southern Virginia, Trump championed a proposed military spending increase as an economic boon.

"American ships will sail the seas. American planes will soar the skies. American workers will build our fleets," Trump said

Katherine Blakeley, a defense analyst at the Center for Strategic and Budgetary Assessments, said that the Pentagon's best hope to authorize a new BRAC round is to tie it to the military strategy Defense Secretary James Mattis is currently preparing, so that BRAC is useful even under a significant military buildup.

"If you frame it right, you might be able to get more support," Blakeley said.

Will Congress go along?

There have been signs Congress could be more open to base closures. In January, Senate Armed Services chairman John McCain of Arizona said he was "seriously considering the issue."

But after the budget was released last week, McCain was much more concerned with boosting military spending than a possible BRAC round, which includes upfront costs to generate greater savings later.

"I don't care what you do about BRAC, if you don't spend the money to get our military back in shape, it doesn't matter," McCain told reporters. "We'll consider what they have to say about it. That's their proposal. But what should I consider when they're at least $30 billion short of what we need to restore the military."

The Pentagon has argued that a new round of base closures and realignments is a "very significant opportunity to get some savings." John Roth, who was the acting Pentagon comptroller during last week's budget rollout, told reporters the plan could generate about $2 billion a year in savings once the costs are recouped.

"The sum total of the four or five rounds that we've had since the 1990s and 2005, has resulted in approximately $12 billion a year in savings by having done that," Roth said. "That is a gift that keeps giving."

There's a bipartisan consensus in Congress, however, that the savings won't materialize. Congress roundly rejected a new BRAC round the final five years it was proposed in the Obama administration. Lawmakers frequently point to the last BRAC round in 2005, which cost billions more than initially estimated, as a key reason not to do it again.

Last year, the Pentagon submitted a report saying it would have 22% excess infrastructure by 2019, but many Republicans dismissed the finding because they believed the military's size should be bigger than what was estimated.

House Armed Services chairman Mac Thornberry, who has a key role in deciding the fate of BRAC, has disputed the Pentagon's data to support a new round of closures.

"They give us the data that shows we need it, and then of course we will consider it," said Thornberry, who is also pushing for a bigger Pentagon budget. "I think the secretary is sensitive to that point."

Sen. Jim Inhofe, an Oklahoma Republican who leads a key armed services subcommittee, also dismissed the need for closing bases.

"The first thing that's a certainty about any BRACs is they cost money in the first three years," Inhofe told Defense News. "We've never been in a position where we have been so undermanned and undersized. The budget's inadequate, and this isn't the time to obligate more funding."

Democrats are also largely opposed. Democratic Sen. Jeanne Shaheen of New Hampshire said she thought a BRAC proposal had "no chance" of being approved by Congress.

"At least in talking to my colleagues, I haven't heard an inclination to go forward in this budget year," she said.

One key defense lawmaker who has pushed for a BRAC is Rep. Adam Smith of Washington state, the top Democrat on the House armed services committee. Smith told CNN he's hopeful that the Trump administration could get better results than Obama's on a Republican-dominated Capitol Hill.

"A Republican president proposed it and we have a Republican Congress, so I think that gives us a better chance," Smith told CNN.

Republicans certainly aren't rushing to embrace BRAC now that Trump is proposing it, but some lawmakers are opening the door to consider reducing Pentagon infrastructure, even if it isn't a full-blown BRAC round.

Rep. Rob Wittman of Virginia said there was consideration in this year's defense authorization bill to authorize the military to conduct a detailed analysis of its infrastructure, building by building.

"It's short of a BRAC, but at least lays out -- OK Congress, what should you do?" Wittman said.

And Rep. Mike Turner, an Ohio Republican, said that a BRAC round may actually be needed once the budget caps that have constrained defense spending in recent years are removed.

"We've gone through the haphazard cuts of sequester that we're probably to the point of needing an organized process to review facilities, and that includes Congress at this point," Turner said.