Four months ago, Thomas Homan, a former director of Immigration and Customs Enforcement under Donald Trump, took to the stage at the Republican National Convention to issue a warning to immigrants living in the country illegally.

“As a guy who spent 34 years deporting illegal aliens, I have a message for the millions of illegal aliens that President Biden released in our country in violation of federal law,” he said. “‘You better start packing now.”’

Less than a week after Trump defeated Vice President Kamala Harris, the Republican president-elect announced that he plans to appoint Homan as his “border czar,” one of many signs Trump will act swiftly and decisively on his campaign promise to secure the border and clamp down on illegal immigration.

Trump, who made mass deportations a centerpiece of his 2024 election campaign, is also expected to appoint another longtime ally and immigration hard-liner, Stephen Miller, as deputy chief of staff for policy.

Homan and Miller are just two of the intended appointments that show Trump is setting up to deliver on the key platforms of his campaign. On Monday, he also named former New York Rep. Lee Zeldin to head the Environmental Protection Agency.

Miller, a former speechwriter and campaign advisor to Trump who worked closely with the former president in the White House during Trump’s first term, was a key architect of Trump’s “zero tolerance” immigration policies, including a travel ban that reduced the number of refugees accepted to the U.S. from Muslim-majority countries and the separation of migrant children from their parents.

“Trump is plainly delivering a message that he intends to do what he said he would do,” said Andrew Arthur, a fellow at the Center for Immigration Studies, a nonpartisan group that supports immigration restriction.

During his campaign, Trump promised to deport millions of immigrants living in the U.S. illegally, carrying out the “largest deportation operation in American history” on Day One. And in his first interview after he won last week, the former president told NBC that he was committed to a mass roundup of undocumented immigrants, whatever the cost.

“It’s not a question of a price tag,” Trump said. “It’s not. Really, we have no choice when people have killed and murdered, when drug lords have destroyed countries. And now they’re going to go back to those countries because they’re not staying here.”

But athough Trump can order a clampdown on undocumented immigrants as soon as he becomes president, few experts expect his administration to immediately deport the millions of people estimated to be in the United States illegally. According to the Pew Research Center, about 11 million immigrants were living illegally in the country in 2022 and more than 2 million people have entered the country illegally since then.

The American Immigration Council, a nonprofit advocacy group, has estimated that deporting everyone in the U.S. illegally would cost at least $315 billion and take at least a decade.

Still, Trump’s crackdown on immigrants has the potential to create chaos across the nation and economic disruption across many industries that rely on immigrants — particularly in California, home to about 1.8 million immigrants without legal status. In L.A. County, a recent USC Dornsife report found that more than a third of residents are immigrants and about 8% are undocumented immigrants.

“This time around, the Trump’s administration is going to be a lot more prepared,” said Masih Fouladi, executive director of California Immigrant Policy Center. “If we thought they acted quickly in 2017, with a Muslim ban a week after Trump was sworn into office, it’s going to be even faster this time. … Trump is going to stay true to his word, advancing mass deportation on Day One, and we similarly have to work with that kind of urgency to make sure that we’re protecting our communities.”

Immigrants living in California and other blue states already benefit from protections that immigrants in red states do not have. For example, California has already established a series of policies that limit local agencies’ cooperation with federal immigration officials.

But Fouladi said immigrants and their supporters had to work immediately with state and national partners to prevent harm to immigrants and their families. He urged the state to strengthen the protections of the California Values Act and invest in rapid response networks and legal services for immigrants who might be deported.

Former White House aide Stephen Miller speaks at a campaign rally in October 2024 for Donald Trump in State College, Pa.

(Alex Brandon / Associated Press)

“There are a lot of mixed families in California, where there might be certain parents that are undocumented, but children that are U.S. citizens,” Fouladi said. “Plans for those families to be taken care of should be one of the top priorities of state lawmakers, the governor, local mayors.”

Fouladi said California leaders had already made provisions to defend immigrants, but they were not working with the same urgency as the Trump administration.

Although Trump has claimed that “many” of the migrants who entered the U.S. illegally over the last 3½ years “are murderers, drug dealers, people from jails,” research shows people living in the U.S. illegally are arrested at significantly lower rates for violent, drug and property crimes than native-born Americans.

Fouladi said he hoped key leaders in California agriculture, industry and chambers of commerce would highlight the vital role immigrants have played in boosting the state’s economy.

“We need urgency in advance of Trump’s taking office,” Fouladi said. “I think this time around folks believe he’s going to advance policies that deport individuals, I just don’t think they appreciate the scope and scale.“

When Trump announced Sunday on his platform Truth Social that he would make Homan “border czar,” he also said Homan would be in charge of “all Deportation of Illegal Aliens back to their Country of Origin.”

In July, according to the news website Semafor, Homan told a panel on immigration policy during a conservative conference: “I will run the biggest deportation force this country has ever seen.”

On Monday, Olivia Troye, a former White House national security official under Trump, warned that Homan was a divisive figure.

When deadly wildfires swept through Northern California in 2017, she noted, Homan spread false claims that an undocumented immigrant had started them and blamed the Sonoma County sheriff for so-called sanctuary policies that “left their community vulnerable to dangerous individuals and preventable crimes.

“The immigrant had NOTHING to do with the wildfires,” Troye posted on X. “Those of us working at DHS who were actually committed to homeland security watched this with horror internally as it played out. Remember this going forward when things like this happen again.”

Miller has said that a Trump administration could deploy the U.S. military and National Guard troops to assist in deporting immigrants, setting the stage for showdowns between the federal government and local jurisdictions that bill themselves as sanctuary cities and states.

“If President Trump gets reelected, the border is going to be sealed, the military will be deployed, the National Guard will be activated, and the illegals are going home,” Miller told conservative podcast host Charlie Kirk this year.

Last month, Trump said at a campaign rally that he would invoke the Alien Enemies Act, a rarely used 1798 law to deport anyone who is not in the country legally. But legal experts believe such a policy would face protracted legal battles.

Arthur said he expected the Trump administration to prioritize the completion of its southern border wall and reinstate its “Remain in Mexico” policy that forced people to stay in Mexico after applying for asylum in the United States — actions that would deter migrants from trying to enter the country.

To fund completion of the border wall, Arthur expected Trump would declare a border emergency or draw upon military assets or go to Congress for a special supplemental appropriation. But even then, Arthur said, it would be impractical to expect immediate results.

“We’re not going to have a fence that stretches from Brownsville to Imperial Beach, but you’re definitely going to be plugging the gaps in key areas of Arizona, California and Texas,” he said.

At the same time, Arthur said, the Trump administration would prioritize returning migrants who have committed serious crimes who are deemed to pose a threat or those who have entered illegally after November 2020.

Asked whether law-abiding immigrants without papers should fear they might be deported if law enforcement pulled them over in a traffic stop, Arthur said: “I’m not going to say they shouldn’t be worried.”

“If they’re encountered, they are going to be taken into custody,” he said. “But are you, all of a sudden, going to go out and try to arrest 13 to 15 million people? That’s a resource issue, and it’s probably not feasible.”