President Trump visits the border in Calexico, Calif., in early April. (Jacquelyn Martin/AP)

The White House sent Congress a $4.5 billion emergency spending request on Wednesday, citing an unfolding “humanitarian and security crisis” at the U.S.-Mexico border as record numbers of Central American families and children seek entrance to the United States.

The request includes $3.3 billion for humanitarian assistance and $1.1 billion for border operations, and it represents a dramatic escalation of the administration’s efforts to address the situation at the border.

The money would be in addition to the more than $8 billion that President Trump asked for in his 2020 budget request to build border barriers, as well as some $6 billion in funding he sought as he declared a national emergency at the border earlier this year.

“The situation becomes more dire each day,” White House acting budget director Russ Vought wrote in the request to congressional leaders. “The migration flow and the resulting humanitarian crisis is rapidly overwhelming the ability of the Federal Government to respond.”

An accompanying fact sheet described the situation at the border in even more dire terms. “This crisis is threatening lives on both sides of the border and is unlike anything we’ve ever seen,” it says.

Democrats responded with skepticism to the request, which comes as Trump prepares to run for reelection on a hard-line immigration agenda. It also arrives as Congress in the midst of a fight over a different emergency spending bill, for disaster aid.

House Appropriations Committee Chairwoman Nita M. Lowey (D-N.Y.) said she would review the request, but she criticized it as an attempt to expand detention of immigrants by the Immigration and Customs Enforcement agency.

“The Trump administration appears to want much of this $4.5 billion emergency supplemental request to double down on cruel and ill-conceived policies, including bailing out ICE for overspending on detention beds and expanding family detention,” Lowey said in a statement. “Locking up people who pose no threat to the community for ever-longer periods of time is not a solution to the problems at the border.”

Vought said the Health and Human Services Department is likely to run out of money to provide child welfare services at the border in June. If that happens, the agency will have to divert critical resources from other programs, will cancel or scale back any services not necessary for protection of human life, and will be forced to leave children in Department of Homeland Security detention facilities where they are not supposed to stay for longer than 72 hours.

“In the worst-case scenario, thousands of children might remain for lengthy periods of time in facilities that were never intended to be long-term shelters,” Vought wrote.

The request also includes $377 million for the Pentagon and National Guard for their operations along the border.

The administration request describes what many lawmakers of both political parties have come to agree is a true crisis at the border, where arrivals of unaccompanied children and families from Central America have spiked dramatically. From October through March, DHS has apprehended more than 360,000 migrants illegally crossing the border — 187,000 more than the same period in the previous fiscal year, the administration says.

At the current rate, DHS apprehensions will approach 1 million in fiscal year 2019. And the demographics of people arriving at the border have shifted strikingly, from individual men to children and families. U.S. facilities are not prepared to accommodate that change. The number of unaccompanied children referred to HHS this year is already 50 percent higher than the same period last year.

The emergency spending request is the latest move by the administration as it attempts, thus far unsuccessfully, to address the grim situation. Earlier this week, Trump ordered major changes to U.S. asylum policies, banning those who cross the border illegally from getting work permits and giving courts a 180-day limit to process asylum claims that can take years under current backlogs.

Political disputes in Congress make a legislative solution appear impossible, forcing the administration to look for other avenues. Trump routinely blames Democrats for this gridlock, although the GOP has blocked past attempts at overhauling immigration laws. Now, the human drama at the southern border threatens to undermine Trump’s efforts to claim success at his signature issue of clamping down on illegal immigration.

The supplemental request would expand the Trump administration’s ability to detain migrants in provisional border facilities and longer-term immigration detention centers.

Trump has been clamoring for funding to detain immigrants, and officials have been exploring possibilities in California, Illinois and other states. Congress rebuffed the administration’s previous efforts to dramatically increase detainees, which have hovered at an all-time-high of more than 45,000 migrants a day. The current budget provides for 42,774 adult beds.

The emergency supplemental request would add more than $300 million to fund an average of 51,300 beds this fiscal year, and allows ICE to ratchet that up to 54,000 by the end of the fiscal year. The money would also fund an additional 960 beds at the Dilley Family Residential Center in South Texas, a sprawling detention camp for mothers and children. The current budget provides for 2,500 beds at that facility.

The Health and Human Services Department’s Office of Refugee Resettlement, which oversees care of unaccompanied minors, is housing nearly 13,000 until they can be placed with a parent or guardian in the United States. The supplemental request would nearly double its capacity to 23,600 beds.

The administration is exploring sites in San Antonio, Phoenix, Houston, Atlanta and Dallas-Fort Worth and last year reopened a massive facility in Homestead, Fla., that can house thousands of children.

House Democrats have called for increased humanitarian funding on the border, but they are certain to fight any expansion to immigration detention that could target undocumented families with deep roots in the country. During the government shutdown this past winter, Democrats attempted to cap the number of detention beds and said their goal was to reduce the numbers over time.

The bulk of the $3.3 billion for humanitarian assistance in the spending request — $2.8 billion — would be used to shore up the budget of the Office of Refugee Resettlement, which oversees the network of shelters where minors are sent after they are processed by DHS.

Caring for migrant children and holding them until they can be released to a sponsor is among the most costly and complicated elements of the U.S. immigration system. The Trump administration temporarily operated a tent camp for older teens in Tornillo, Tex., but closed the controversial facility earlier this year, and continues to struggle to find shelter operators capable of taking in large numbers of migrant children.

Costs of care are extremely high. Adult migrants can be detained in immigration jails at relatively inexpensive rates, but the facilities where minors are sheltered have trained staff, recreational and educational programming and significant liabilities. At some facilities, the cost of sheltering a child exceeds $750 per day.

Minors typically remain in the shelters until the government can identify an adult sponsor who can take custody, often one of the child’s parents already present in the country. The average time that it takes to complete the sponsorship process is 66 days, according to the latest HHS figures.

The emergency funding request echoed a similar ask from the Obama administration in 2014 after a surge of families and unaccompanied children from Central America overwhelmed Border Patrol stations. The Obama White House requested $3.7 billion, including $1.8 billion for HHS, and $1.5 billion for DHS, but congressional Republicans blocked efforts to approve additional funding.

“They’re not even trying to actually solve the problem,” President Barack Obama said at the time.

The number of Central American families seeking to enter the United States this year already has soared well past the totals from the 2014 border crisis.

Seung Min Kim and David Nakamura contributed to this report.