“I can’t even begin to picture how we would deport 11 million people in a few years where we don’t have a police state, where the police can’t break down your door at will and take you away without a warrant,” said Michael Chertoff, who led a significant increase in immigration enforcement as the secretary of Homeland Security under President George W. Bush.

Finding those immigrants would be difficult, experts said. Police officers across the country would need to ask people for proof of residency or citizenship during traffic stops and street encounters. The Border Patrol would need highway checkpoints across the Southwest and near the Canadian border. To avoid racial profiling, any American could expect to be stopped and asked for papers.

To achieve millions of deportations, the Obama administration’s focus on deporting serious criminals would have to be scrapped, said Julie Myers Wood, a director of Immigration and Customs Enforcement, also known as ICE, under Mr. Bush. “You would not care if the person had a criminal record,” she said.

Large-scale raids, rare under Mr. Obama, would resume at farms, factories, restaurants and construction sites, with agents arresting hundreds of workers and poring over company records. And prosecutors would bring criminal charges against employers hiring unauthorized immigrants.

Mr. Trump has said he would triple ICE’s deportation officers, to 15,000 from about 5,000. But even if that could be accomplished quickly — difficult given the vetting and training required — it would still be insufficient, experts said. The F.B.I. and other agencies would have to set aside some of their missions to help.

John Sandweg, who led ICE for seven months under Mr. Obama, said wholesale deportations could make it easier for immigrant gang members and drug traffickers to escape detection. “If the agents are looking for volume, they won’t spend the time to do the detective work tracking down the high-value bad guy who has fake documents, the hardened criminals in the shadows,” he said.

To prevent flight after arrest, the authorities would have to detain most immigrants awaiting deportation. Existing facilities, with about 34,000 beds, would have to be expanded to hold at least 300,000, Mr. Sandweg estimated, perhaps with tens of thousands of people in detention camps, similar to the internment of Japanese-Americans during World War II.