(CNN) Donald Trump has vowed to kick off his presidency by swiftly kicking out one segment of the immigrant population: criminals.

But that's exactly what the Obama administration says it's been doing for eight years. More than 2.4 million people have been deported under President Barack Obama, earning him the scathing nickname of "deporter in chief" among some immigrant advocacy groups. Nearly half of those removals were immigrants with criminal convictions, an explicit priority of the administration.

Though Trump made cracking down on illegal immigration a cornerstone of his campaign, critics question whether he'll be able to accelerate deportations much. Some experts say there are bottlenecks in the system that prevented Obama from deporting more immigrants.

Obama and Trump clearly don't see eye to eye on many aspects of immigration. Obama backs a pathway to citizenship. Trump plans to build a wall on the southern border. But there's one area where they agree. When it comes to the issue of deporting criminals, even their rhetoric sounds similar.

Here's Obama's take on whom authorities should focus on deporting:

Felons, not families. Criminals, not children. Gang members, not a mom who's working hard to provide for her kids." President Barack Obama in 2014

What we are going to do is get the people that are criminal and have criminal records, gang members, drug dealers." President-elect Donald Trump on "60 Minutes"

A NEW APPROACH?

But Trump and his supporters accuse Obama of deliberately dragging his feet.

and kicking out immigrants who are criminals and should have been deported already. And The President-elect says his focus will be tracking downand kicking out immigrants who are criminals and should have been deported already. And he says there could be up to 3 million. "We're getting them out of our country. They're here illegally," he told "60 Minutes."

Trump's number is higher than other figures analysts have used. The nonprofit Migration Policy Institute estimates 820,000 undocumented immigrants have criminal convictions

So where does the statistic Trump's using come from? Kansas Secretary of State Kris Kobach, who's advising Trump on immigration, says the President-elect was looking at a Department of Homeland Security report from 2013 , which used census figures to estimate there could be up to 1.9 million "removable criminal aliens" in the United States. "That report's 3 years old, so President-elect Trump would be saying, 'It's probably gone up,' " Kobach told CNN

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DEFINING CRIMINAL

Official reports on deportations describe immigrants with convictions as criminal aliens. But exactly what that term means depends on whom you ask

Data about whom the US government has deported in recent years shows that the term can apply to immigrants who've committed a wide range of crimes. Government data obtained by Transactional Records Access Clearinghouse, or TRAC, a data research group at Syracuse University, found the two most prevalent crimes of those removed in recent years were immigration and traffic offenses. Drug crimes came in third.

"Criminal aliens" can be undocumented immigrants or lawful permanent residents. They may have gotten caught up in the court system for a shoplifting offense -- or served lengthy prison sentences for something much more severe.

As officials trumpet the priority of kicking out criminals, why would traffic offenses take up such a large share of the criminal deportations on the books?

"Going after those with 'traffic-related' offenses helped (immigration officials) at least nominally look as if they were achieving their announced goals of deporting serious criminals from our midst," says Susan Long, co-director of TRAC.

Another possibility: Authorities are using the courts to prosecute immigration law violations more than they used to do.

"Formerly, individuals were deported administratively, and infrequently sent through the criminal justice system," Long says.

Immigrations and Customs Enforcement told CNN that the agency changed priorities in 2014. Under current policies, traffic offenses and local immigration offenses are not a priority for deportation. As a result, ICE says it is now better targeting people who pose a national security or public safety threat.

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A looming question: Where will Trump draw the line?

"I think you'll see probably a Trump administration saying, 'Look, we're going to define criminal broadly and not so narrowly,' " Kobach told CNN this month.

OBSTACLES LOOM

No matter what guidelines Trump uses, it's likely deporting millions of people won't be as simple as it sounds.

"We're not going to tear families apart. So we will do everything we know how to do to resist that," New York Mayor Bill de Blasio said after Trump's election

Trump has vowed to fight back with federal funding cuts. No matter what happens next, sparring over the matter is likely to slow any massive deportation efforts.

And immigration courts are already backlogged, with more than half a million cases on the docket and fewer than 300 judges to hear them.

The average wait time for a case in immigration court is 670 days -- nearly half the length of a presidential term.

Michelle Mittelstadt of the Migration Policy Institute says that deportations are expensive, and without substantially new funds from Congress, the system can't handle much more.