In this June 22, 2016 file photo, a Border Patrol agent walks along a border structure in San Diego, Calif. The Trump administration has proposed spending $18 billion over 10 years to significantly extend the border wall with Mexico. Credit: Gregory Bull | AP

WASHINGTON – The Trump administration has told lawmakers that it wants $18 billion over the next decade for the initial phase of a Mexico border wall, laying out for the first time a detailed financial blueprint for the president’s signature campaign promise.

The money would pay for 316 miles of new fencing and reinforce another 407 miles where barriers are already in place, according to cost estimates sent to senators Friday by U.S. Customs and Border Protection. If the work was completed, more than half of the 2,000-mile border with Mexico would have a wall or other physical structure by 2027.

Democratic lawmakers blasted the $18 billion request, first reported by the Wall Street Journal, and it arrived in the middle of delicate budget negotiations that include the risk of a government shutdown Jan. 20 if no deal is reached.

“President [Donald] Trump has said he may need a good government shutdown to get his wall. With this demand, he seems to be heading in that direction,” said Sen. Richard Durbin of Illinois, the ranking Democrat on the Senate Judiciary Committee’s immigration subcommittee.

CBP provided the funding outline at the request of Durbin and other senators preparing to launch negotiations this month on several contentious immigration issues, including a potential deal to protect the hundreds of thousands of young immigrants who will be subject to deportation when the Deferred Action for Childhood Arrivals (DACA) program expires, beginning in March.

With their votes needed to keep the government open, Democrats are looking to use their leverage in the spending talks to force the Republicans who control Congress to reach a deal on DACA.

Though Trump ran for office on a promise that Mexico would pay for a border wall, the spending plan indicates American taxpayers would fund it for at least the foreseeable future.

Trump has told Democrats he is unwilling to reach an agreement unless they fund his wall plan, among other measures, but the CBP document is the first time his administration has sketched out what that might cost.

In addition to the $18 billion in wall funding, the CBP also requested $8 billion for additional personnel and training, $5 billion for new border technology and at least $1 billion to build more access roads. The final price tag for the CBP spending plan would exceed $33 billion over the next decade, according to a copy of the document obtained by The Washington Post.

The $33 billion would not include what are likely to be additional funding requests for the other Department of Homeland Security agencies central to Trump’s plans for an immigration overhaul, including Immigration and Customs Enforcement (ICE), which is looking to add 10,000 more officers and dramatically expand the number of beds it has available for immigration detention.

Benjamin Cassidy, the DHS assistant secretary for legislative affairs, said in a letter to Durbin that the funding requests “were developed through a rigorous assessment and are derived directly from the experience and insight of U.S. Border Patrol Agents in the field, supported by operational data and analysis.”

“It is essential to note that this submission represents only one element of the President’s overall immigration priorities,” Cassidy wrote. “Effective border security will not be successful unless we close dangerous legal loopholes that enable illegal immigration and visa overstays. If these loopholes are not closed, and enforcement capabilities are not enhanced, our immigration system and border cannot be secured.”

Democrats have repeatedly said they will not pay for a wall. Even though on a year-over-year basis the CBP request would not represent a dramatic funding increase over current border spending levels, it would represent a long-term commitment to a physical structure that Trump would be able to claim as a political trophy.

Democrats and immigration activists have recoiled at the administration’s other enforcement plans, including proposals to expedite the deportation of unaccompanied minors, tighten asylum standards and crack down on so-called “sanctuary cities” that do not allow local police to cooperate with federal immigration agents.

“Trump can have a shutdown fight over his stupid wall that pleases the nativists in his base, or he can have a breakthrough that pairs the Dream Act and border security so he can brag he did something Obama couldn’t get done. He can’t have both,” said Frank Sharry, executive director of America’s Voice, an immigrant advocacy group. “This move by the White House does not bode well.”

Republican reaction was muted Friday. Even many Republicans have deemed a physical wall along the entire border unnecessary and impractical, saying that more effective border security could be achieved through other means, including better surveillance and technology.

CBP said it would like to add more than 2,000 miles of what it called a “total Border wall system” that would eventually encompass 864 miles of new primary wall on land where no barrier currently exists. It would add 1,163 miles of replacement wall or secondary wall, which DHS officials said could consist of a wall backed by a fence or two fences.

Some of the new barriers would have to be installed along the winding banks of the Rio Grande, where landowners have protested construction of new fencing on their ranches and farms.

The $18 billion would cover only the initial phase of the CBP plan. About 650 miles of the border with Mexico currently has some form of physical obstacle, from vehicle barriers to taller steel fencing designed to prevent people from climbing over.

CBP is evaluating several prototypes, all of which are significantly taller, but the funding request sent to senators does not specify what type of barrier would be used.

Trump has said he may travel to San Diego to inspect the prototypes, which range in height from 18 to 30 feet and combine different formulations of see-through steel bars, concrete slabs and metal spikes.

The Washington Post’s Ed O’Keefe contributed to this report.