If Mr. Trump targeted only those with serious offenses, the number becomes even smaller. About 690,000 of the 820,000 were convicted of felonies or serious misdemeanors.

There are about 820,000 unauthorized immigrants who have been convicted of crimes , according to the Migration Policy Institute, a nonpartisan research group.

But that number included more than a million immigrants who were here legally, with green cards or temporary visas. Unauthorized immigrantsLegal immigrants

Mr. Trump may be basing this number on a 2012 government estimate that there were 1.9 million “removable criminal aliens.”

After the election, Mr. Trump vowed to quickly deport two million to three million unauthorized immigrants who he said have been convicted of crimes. The Department of Homeland Security on Tuesday announced details to expel immigrants more aggressively.

The tiny figures here represent the estimated 11 million immigrants who are in the United States illegally. Early in the campaign, Donald J. Trump said he would deport all of them. (Each figure represents about 5,000 people.)

It’s unclear if Mr. Trump could carry out the deportations he has proposed without violating due process, especially at the scale and speed he has suggested.

The last time the United States carried out mass deportations was when President Dwight D. Eisenhower authorized military-style roundups to expel hundreds of thousands of Mexicans in 1954.

What Mr. Trump proposes is much larger than Eisenhower’s deportation program, which was controversial at the time.

Mr. Trump would likely need help from local law enforcement, but many cities and counties have limited their cooperation with federal authorities.

Counties with policies limiting cooperation

WASH. MINN. ORE. MASS. N.Y. WIS. R.I. CONN. IOWA PA. N.J. NEB. NEV. ILL. MD. COLO. CALIF. KAN. KY. N.M. GA. LA. TEX. FLA. WASH. ORE. MINN. MASS. N.Y. WIS. R.I. CONN. IOWA PA. NEB. N.J. NEV. ILL. MD. COLO. KAN. KY. CALIF. N.M. GA. TEX. LA. FLA. WASH. ORE. MINN. MASS. N.Y. WIS. R.I. CONN. PA. IOWA NEB. NEV. N.J. ILL. MD. COLO. KAN. KY. CALIF. N.M. GA. TEX. LA. FLA.

Cook County in Illinois, which includes Chicago, adopted an ordinance that generally bars county police officers and jails from detaining unauthorized immigrants to be turned over to federal agents, unless the agents have specific warrants.

San Francisco is a so-called sanctuary city that issues local ID cards to unauthorized immigrants so they can have access to municipal services.

Democratic mayors of some major cities recently said they would do all they could to protect residents from deportation.

Mr. Trump’s plans are more aggressive than President Obama’s deportation program, which prompted sharp criticism from advocacy groups. During Mr. Obama’s first term, about 400,000 immigrants were deported per year.

Deportations by year

400 thousand Immigration violators 300 300 200 100 Convicted criminals 35% 59% FY ’09 FY ’15

The Obama administration has already pursued policies that focus on removing people with criminal records. In 2015, the majority of people deported were convicted criminals.