Defense Secretary Jim Mattis is considering a request from the Department of Homeland Security that he spend about $450 million to build part of President Trump’s border wall in Arizona.

But the construction itself could run headlong into time-consuming environmental regulations and the funding may require support from a Congress that is bitterly divided over Trump, the border and immigration.

The Pentagon provided no timeline for Mattis’ decision on Friday. Homeland Security Secretary Kirstjen Nielsen sent the request last month after announcing in April that the agency was looking into ways the military could help complete the president’s promised wall.

“We're going to have our wall, and we're going to get it very strongly. The military's going to be building some of it,” Trump said at the time.

[Related: Trump 'willing to do anything' to secure border wall funds]

The request includes constructing a 30-foot barrier along nearly 32 miles of Arizona's Air Force’s Barry M. Goldwater bombing range near Yuma. The section, which now has pedestrian fencing, would also feature a patrol road and access gates, Lt. Col. Jamie Davis, a Pentagon spokesman, said.

The planned barrier and gates “have proven successful along other parts of the southern border,” Davis said in a statement. Overall, the U.S.-Mexico border is nearly 2,000 miles long.

The Navy is doing advance planning and an initial environmental survey of the Air Force land, which includes looking for unexploded ordnance along the bombing range.

The survey could prove key because the project falls under the regulations of the National Environmental Policy Act, according to Naval Facilities Engineering Command.

That means if the Navy finds evidence a new 30-foot barrier could have a significant impact on the surrounding environment, the military will be required to conduct a much more thorough assessment and likely put Trump’s border wall plan out for public comment.

However, the Navy could also decide the wall is exempt from further regulations after its initial survey if it meets one of 45 categorical exclusions and is not seen as an environmental threat.

But the estimated $450 million cost might still pose a tough challenge.

Mattis could take the funding from the Pentagon’s existing budget for military construction. Congress would need to approve a reprogramming request for the money and it could be a tough sell to lawmakers who face seeing other construction projects being cut at bases in their districts and states.

Another option would be requesting the money through Congress’ armed services and appropriations committees. A $450 million project would need its own line item in the 2020 defense budget, but that would likely draw a political fight.

House Democrats including Rep. Adam Smith, D-Wash., the ranking member on the House Armed Services Committee, blasted the price tag this week in a letter to Mattis.

“In countless hearings and briefings this year we have heard from senior civilian and military leadership on the readiness challenges our military currently faces,” Smith and Democrats on the House Appropriations Committee wrote to Mattis. “With that in mind, we fail to see how diverting $450 million away from efforts to rebuild military readiness is in the Department or the taxpayers’ best interests.”

There are two emergency options open to Mattis and Trump if they need to sidestep funding roadblocks in Congress.

Mattis could declare the 32-mile section of wall vital to national security. Under federal law that would allow him to spend up to $50 million per year on building it.

Mattis and Trump could also ignore funding limitations and environmental laws if the president declares a national emergency over the project, though the money would still have to come out of military construction coffers, according to federal law.