As the 2020 presidential election gets into full swing, President Donald Trump is focusing on one of his favorite topics: immigration.

Over the past several days, the president has warned he'll cut off hundreds of millions of dollars in aid to Central American nations where migrants come from, and he threatened to seal off the entire southern border. That follows his decision to declare a "national emergency," so he can expand the border wall without approval from Congress.

All that is happening as the United States tries to renegotiate its trade agreement with Mexico and Canada.

"Mexico must use its very strong immigration laws to stop the many thousands of people trying to get into the USA," Trump tweeted Monday morning. "Our detention areas are maxed out & we will take no more illegals. Next step is to close the Border! This will also help us with stopping the Drug flow from Mexico!"

Tom Jawetz, vice president for immigration policy at the liberal Center for American Progress, said the president's actions represent a purely political strategy to create the impression of chaos along the border.

He said Trump "needs there to be chaos, so he can have something to fight against."

Migrant families coming in record numbers

The Trump administration pointed to a massive increase in the number of people trying to cross the U.S.-Mexican border in recent months as its reason for declaring a national emergency.

Overall, illegal immigration across the southern border remains lower than the peak years throughout the 1990s and 2000s, when the Border Patrol regularly apprehended more than 1 million undocumented immigrants a year crossing the southern border.

The makeup of people crossing that border has changed in recent years. Back then, single, Mexican males looking for work and trying to evade Border Patrol agents made up the majority of undocumented border crossers. Now, most crossers are Central American families seeking out Border Patrol agents and turning themselves in to request asylum.

In February, Border Patrol agents apprehended 66,450 people illegally crossing the southern border. A record high 36,174 of those (54 percent) were members of families and 6,825 (10 percent) were unaccompanied minors, according to Border Patrol data.

Those numbers have steadily increased in recent months, and Trump administration officials said the number of illegal crossings in March will be even higher.

Trump wants to cut off aid to Central America

Thursday, Department of Homeland Security Secretary Kirstjen Nielsen met with her counterparts in El Salvador, Honduras and Guatemala to sign a "historic" agreement for the four countries to cooperate on improving the security situation in the region.

By the next day, Trump had announced he would cut off all foreign aid to those countries.

The president had threatened such a move since he first started tweeting about migrant caravans last year during the lead-up to the 2018 elections. Trump accused the leadership of El Salvador, Honduras and Guatemala – the "Northern Triangle" countries where most migrant families come from – of not doing enough to stop their people from heading to the USA.

The administration had been slow-walking millions of dollars of contracts authorized by Congress awarded to local government agencies, nonprofit organizations and U.S. officials to work on improving the economic and security situations in those countries. Friday, Trump announced he will try to cut off entirely more than $500 million in aid dedicated to the Northern Triangle in the 2019 fiscal year.

It's unclear whether Trump can cut off all that money without the help of Congress. .

"American aid to Central America is not charity, but an investment in our national security – full stop!" wrote U.S. Global Leadership Coalition President and CEO Liz Schrayer. "The idea of suspending the relatively small, but essential foreign assistance to the region will only exacerbate the root causes driving people to flee their homes – brutal violence, hunger, and instability."

Trump threatens to seal southern border

If Trump can cut off foreign aid to Central America, he may follow through on another oft-repeated threat: to seal off the entire southern border.

Nearly 50 crossings along the U.S.-Mexican border would be closed for entry and exit, stopping the hundreds of thousands of people and about $1.7 billion in goods and services that cross each day, according to the State Department.

Trump wouldn't be the first president to limit crossings along the southern border.

President George W. Bush partially closed the southern border after the 9/11 terrorist attacks, requiring full inspections of every incoming pedestrian and vehicle that led to days-long waits. President Ronald Reagan ordered similar restrictions in 1985 after the kidnapping and murder of a DEA agent in Mexico.

Those closures were not nearly as controversial, and Trump's move would probably draw immediate lawsuits, as many other of his immigration enforcement actions have faced in the past two years.

Border Patrol is shorthanded

Further complicating the situation along the southern border is the fact that the Border Patrol is trying to manage the increased flow of families as it's struggling to hire, and simply retain, agents.

During his first week in office, Trump ordered the agency to hire an additional 5,000 Border Patrol agents. Internal watchdog reports show that the agency probably won't meet that demand because of difficulties finding qualified applicants and internal mismanagement that hurt its efforts to hire more people.

The result: In 2018, the agency added 118 agents, only three of which were stationed along the southern border.

That partly explains why Trump has deployed thousands of National Guardsmen and active-duty military troops to the southern border.

Customs and Border Protection Commissioner Kevin McAleenan announced last week that he would reassign 750 customs officers from ports around the country to help process asylum-seeking families along the southern border. Monday, DHS Secretary Nielsen said that number could surge to 2,000 if necessary.

Releasing migrant families on U.S. streets

During his trip to El Paso, Texas, last week, McAleenan said the border had reached its "breaking point," forcing his agency to use extreme measures to keep up.

One of those changes has been to start quickly releasing migrants into the streets of border communities, breaking with the administration's practice of detaining them as long as possible.

Under U.S. law, the Border Patrol is not supposed to hold any migrant longer than 72 hours. Usually, the Border Patrol hands them over to Immigration and Customs Enforcement, which can detain families for up to 20 days. All of those facilities are overcrowded, leading the Border Patrol to skip the transfer to ICE and release migrants to shelters en masse.

From Arizona to Texas, and community organizations have stepped in to help the stranded migrants.