WASHINGTON – President Donald Trump says his administration wants to pursue a "tougher direction" on immigration, and the president is overseeing a leadership purge at the top levels of the Department of Homeland Security to make it happen.

But what does a "tougher direction" mean? And does the president have new options following the resignation Sunday of Homeland Security Secretary Kirstjen Nielsen?

For some advocates, it's about a cultural change.

"Within DHS, there have been a lot of policy suggestions, small and large, that have been worked up but have been blocked by the people who are the heads of the agencies," said Roy Beck, president of NumbersUSA, a group that advocates for lower levels of legal and illegal immigration.

Start the day smarter:Get USA TODAY's Daily Briefing in your inbox

Nielsen's departure:After resigning, Kirstjen Nielsen says she supports Trump's goal of 'securing the border'

Nielsen:Homeland Security Secretary resigns amid migrant surge at US-Mexico border

A look at some of the ideas that may be on the table:

Return to 'zero tolerance'

Immigration advocacy groups were bracing Tuesday for the possibility that the Trump administration would return to the "zero tolerance" policy that led to the separation of thousands of migrant families.

Then-Attorney General Jeff Sessions issued a memo last year requiring that all immigrants entering the country illegally would be prosecuted. But a 1997 settlement in a federal court case required the U.S. to release children immigrants after 20 days. To honor both commitments, the Trump administration separated children and adults.

Trump backed down under immense pressure, signing an order in June to end family separations. Immigration advocates pointed to reports Monday that Trump and adviser Stephen Miller want to re-start the process with Nielsen gone.

Key moments from Nielsen's tenure:Migrant family separations, heckling

Nielsen resignation:Donald Trump's Cabinet agencies are increasingly led by 'acting' secretaries

"Trump and Stephen Miller's desire to go in a 'tougher direction' means we are heading for an even more dangerous and draconian era," said Kerri Talbot, director of federal advocacy for the advocacy group Immigration Hub.

"More families will be separated while Trump and Miller disregard the law and create chaos in our immigration system," she said.

'Closing' the border

Trump repeatedly threatened to close the border last month before he retreated in recent days, arguing that Mexico had taken a more aggressive stance on apprehending Central American migrants in response to his words.

Closing all ports of entry along the U.S.-Mexico border would be a logistical and economic nightmare, experts say. Roughly $1.5 billion in trade takes place across the border every day. But past presidents – including Richard Nixon and Ronald Reagan – took steps to slow traffic through checkpoints to send a political message to Mexico.

Reagan closed nine border crossings and increased inspections at the rest in 1985, a response to the abduction and murder of Drug Enforcement Agency agent Enrique Camarena. Nixon approved Operation Intercept during his first year in office, increasing inspections at U.S.-Mexico border crossings to stem the flow of drugs.

More:'It was long past time for her to go': Dems welcome Kirstjen Nielsen's departure as DHS secretary

More:Who is Kevin McAleenan, Trump's acting homeland security chief after Kirstjen Nielsen leaves?

Mexican auto tariffs

Trump threatened last week to impose a 25% tariff on all cars made in Mexico and shipped into the U.S. if the Mexican government stopped apprehending immigrants who cross the border illegally. If that doesn’t work, he said, then he would close the border.

The U.S., however, already has agreed not to impose tariffs on cars built in Mexico.

A new trade agreement the Trump administration negotiated last year with Mexico and Canada includes a side letter that specifically exempts a certain number of passenger vehicles from any new tariffs the U.S. might impose on autos under the guise of national security.

For Mexico, the agreement means that 2.6 million passenger cars would be exempted from any new tariffs.

Trump, however, said on Twitter that his latest threat to impose tariffs would “supersede” the new trade pact.

Asylum changes

Trump has sought to limit the number of people seeking asylum, claiming the system is a "scam" and that migrants only use the claims to get a foot in the door before disappearing rather than going through the process of confirming their eligibility.

Under Nielsen, the Trump administration sought to address that problem by requiring asylum claimants to remain in Mexico rather than entering the U.S. A federal judge Monday blocked that policy, ruling that DHS overstepped its legal authority.

Beck, with NumbersUSA, said the administration may now be considering changes to executive decisions made by past administrations.

More:Nielsen resignation doesn't change fact child sexual abuse at border is real emergency

Among those, for instance, would be deputizing Customs and Border Patrol agents to take a more substantial role in deciding whether asylum claimants have a credible case to remain in the country. Supporters said that move could speed up reviews so migrants who don't meet asylum requirements are returned to their native country more quickly.

"There are some innovative ideas that have to do with way regulations are put together," Beck said ."One of the big knocks against the current DHS secretary is that a lot of proposals have gone to her office and just died there."

Contributing: Alan Gomez