Democrats and immigration activists are firing back at Homeland Security Secretary Kirstjen Nielsen Kirstjen Michele NielsenDHS IG won't investigate after watchdog said Wolf, Cuccinelli appointments violated law Appeals court sides with Trump over drawdown of immigrant protections Democrats smell blood with new DHS whistleblower complaint MORE, blaming her for the humanitarian crisis on the southwest border as the government struggles to deal with a surge in asylum seekers.

Nielsen has painted a dire picture of the situation with federal law enforcement agencies forced to choose between manning ports of entry and caring for asylum seekers crossing between those ports. And President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE raised the stakes on Friday, vowing to close down the border if Mexico does not do more to stop migrants.

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"The system is in freefall. DHS is doing everything possible to respond to a growing humanitarian catastrophe while also securing our borders, but we have reached peak capacity and are now forced to pull from other missions to respond to the emergency," Nielsen said in a statement Friday.

The statement followed a a letter to the House Homeland Security Committee on Thursday, in which Nielsen requested emergency funding for border security and specific powers to detain asylum-seeking family units for longer than currently allowed.

But Democrats are pushing back, accusing Nielsen of misrepresenting the cause of the crisis and failing to understand how to address it.

"In her letter to Congress, the Secretary continues to confuse and mislead on the situation at the border and how we got to this point. President Trump’s border security and immigration policies have failed, and the Administration deserves much of the blame for making the difficult situation at the border worse," said House Homeland Security Chairman Rep. Bennie Thompson Bennie Gordon ThompsonHouse panel pans ICE detention medical care, oversight Senate to hold nomination hearing for Wolf next week Hillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers MORE (D-Miss.) in a statement Friday.

Congressional Hispanic Caucus (CHC) Chairman Rep. Joaquín Castro (D-Texas) piled on after Nielsen's letter, saying the administration "has acted in bad faith when it comes to immigration and asylum policies."

"The Department of Homeland Security policies have exacerbated the humanitarian crisis at the border, and now Secretary Nielsen has put forth a list of proposals that will continue to hurt asylum seekers and deepen the crisis," added Castro in a statement.

Democrats said Trump's threat to shut the border would only make the situation worse.

"Threatening Mexico with a full border closure is just another rash decision, reflective of the president’s obsession with partisan talking points and campaign rallies," said Rep. Vicente Gonzalez (D-Texas) in a statement.

Neither side denies a crisis exists amid large numbers of asylum seekers heading to the border. But there is a sharp disagreement over how the situation on the border reached a tipping point.

The administration is calling it an immigration crisis, while Trump critics say it has been caused by mismanaging resources at the border.

The Trump administration attributes the crisis to sheer numbers -- apprehensions of family units outside ports of entry at the border spiked in February, forcing the Border Patrol and Customs and Border Protection to redirect resources to care for the migrant families.

In February, the Border Patrol apprehended 36,174 people traveling as part of family units and 6,825 unaccompanied children (UACs). In the four months prior, border apprehensions hovered around 25,000 apprehensions of family unit members and 5,000 UACs per month.

But the crisis seems contained to asylum-seeking families, as official figures show that illicit crossing attempts by single adults seem to be following their regular seasonal patterns, maintaining the relatively low numbers seen over the past decade.

In February, the Border Patrol apprehended 23,451 adults not traveling in families, an uptick from the colder months of December and January — 18,479 and 18,678, respectively — but similar to the number seen in October and November, 22,918 and 21,431, respectively.

Immigrant rights activists and Democrats blame the surge in family unit apprehensions on Trump policies implemented mostly under Nielsen.

"The problem was created on her watch and instead of saying this is what I'm going to do and this is how I'm solving it right now, she's crying, 'I need more resources, I need this I need that.' and people are saying, 'no, no, we just gave you some, go do your job,'" said Ur Jaddou, director of DHS Watch, a progressive immigration watchdog.

"In any other world, she would be fired," she added.

Opponents say two DHS policies, known as "metering" and the Migrant Protection Protocols (MPP), hold particular blame for pushing migrants away from legal ports of entry.

Under metering, prospective asylum seekers are given turns to present their claim at ports of entry, often forcing Central American families to wait for weeks in northern Mexican cities.

And MPP, also known as "remain in Mexico," is a program that allows for some asylum seekers to undergo their first screening with U.S. authorities, and then be returned to Mexico while their cases are reviewed by immigration courts -- a process that can take weeks, months, or years.

"They instituted metering a long time ago all over the border. That exasperates the issue exponentially because … people end up spreading out across, in between ports of entry where there are no facilities, you're relying upon the Border Patrol to pick up people, they don't have vehicles, it's complete chaos," said Jaddou, a former chief counsel to United States Citizenship and Immigration Services (USCIS).

"They should've instead had an interagency process ... to discuss, 'how do we build facilities at ports of entry where it can be organized, orderly,'" she added.

Both sides in the debate also have sharply different views on how to manage the root causes leading to the rise in Central American emigration.

Nielsen traveled to Tegucigalpa, the capital of Honduras, this week to sign a security pact aimed at reducing migration through tougher law enforcement in the Northern Triangle -- Guatemala, El Salvador and Honduras.

One of the stated goals of the pact is to prevent the formation of migrant caravans -- a relatively new phenomenon. Advocates for migrants say the caravans provide safety in numbers but others call them an economic boon for smugglers who can charge up to $8,000 per person for the trip.

The caravans though have made clear the problems with a border security force built to catch surreptitious border crossers rather than deal with large groups of asylum-seeking families.

"The system was built to catch people evading, and single young men ... Now it's an easier job because they're coming to you, you don't have to have this large, large border patrol force roaming the desert, now you just place them at ports of entry," said Jaddou.

The left has grown increasingly frustrated with the administration's focus on enforcement — highlighted by a Trump's push for a border wall.

Frank Sharry, director of America's Voice, a liberal immigration group, called the focus on deterrence "stupid."

"It's a complicated problem that requires a complicated response and the simple-minded Trump folks are making this worse," he said.

Immigration advocates like Sharry say reassigning resources from border and interior enforcement to deal with asylum claims would lead to more orderly migration. And they say promoting economic development in Central America address the root causes.

Trump, though, on Friday laid the blame on Mexico and Democrats, who he said has passed "weak immigration laws," as he threatened to close the border.

A senior Homeland Security official told reporters on a call that the closures would come as officers at ports of entry are reassigned to care for asylum seekers in remote areas, not as a way to punish Mexico.

Democrats say they intend to step up their public opposition to Nielsen and the policies she is enacting.

"I think it's fair to say Nielsen has declared open season on immigrants," said Rep. Nanette Barragan (D-Calif.).