"They have to go," Trump told NBC's "Meet the Press."
The plan lacks some important details and appears to conflict with some of Trump's past statements, said Mark Krikorian, the executive director of the conservative Center for Immigration Studies. All the same, Krikorian noted, the plan is the first detailed proposal from any Republican candidate in the 2016 race so far.
"It's a very useful step forward, not just for Trump but for all of the candidates to get this debate away from vacuous generalities," Krikorian said.
Here we try to answer a few questions you might have about Trump's plan.
1. What does Trump mean by ending birthright citizenship?
Since an 1898 ruling by the Supreme Court, the 14th Amendment has guaranteed citizenship to anyone born in the United States, even if the parents are here illegally.
About 350,000 children were born in the United States in 2009 who had at least one parent who was an undocumented immigrant, according to the Pew Research Center. They accounted for about 8 percent of all babies born here that year. Yet many of them likely had one parent who was either a citizen or an immigrant living here legally. Trump's plan does not specify exactly which babies would be denied citizenship.
In 2010, according to Pew, there were a total of 4.5 million people who had been born in the United States to parents who were undocumented immigrants. Trump's plan does not specify whether their citizenship will be revoked.
Trump's campaign did not immediately respond to a request for clarification, but Wonkblog will add more information to this post if and when the campaign makes it available.
Trump's plan also does not describe how he would end birthright citizenship. He would likely have to persuade the Supreme Court to overturn the longstanding precedent. Trump could also propose an amendment to the Constitution.
The nonpartisan Migration Policy Institute has projected that by 2050, ending birthright citizenship for future children would increase the undocumented population to 16 million if citizenship were denied to children whose parents are both here illegally. The figure would increase to 25 million if citizenship were denied to the offspring of at least one unauthorized immigrant. With no change in law, the unauthorized population would remain steady at around 11 million.
"I think that's really a recipe for social disaster in the coming generation. We've seen this in Europe for example," said Hiroshi Motomura, a professor of law at the University of California, Los Angeles. "What you have are large disaffected populations."
Ending birthright citizenship would have some unexpected consequences, Motomura noted. Millions of young Americans would be unable to work legally, reducing the labor force and the overall strength of the economy.
Additionally, many babies could be born without citizenship in any country if the laws of their parents' native country didn't extend citizenship to them. It is hard to know how many would fall into this category.
Krikorian opposes birthright citizenship, but he argues that ending birthright citizenship would only be possible along with amnesty for those undocumented immigrants already in the country so that their children would be citizens. Otherwise, he said, he would be worried about the unauthorized population—unable to work and without any legal connection to their native country.
"It's a lot of people," said Krikorian. "I don't think it's a good idea."
Krikorian said that Trump was the first major presidential contender to endorse ending birthright citizenship. On Monday, Wisconsin Gov. Scott Walker told MSNBC's Kasie Hunt that he agreed birthright citizenship should be ended.
2. Could Trump actually deport everyone who is here illegally?
Amnesty isn't on Trump's agenda, though. His plan is deportation, which Krikorian dismissed as unrealistic.
"It's a gimmick," Krikorian said. "He's just making it up as he goes along. What ever goes into his mind comes out of his mouth. There's no way to deport 11 or 12 million people in a short period of time."
To be sure, ending birthright citizenship wouldn't increase the unauthorized population if a President Trump deported would-be parents before they could have children here.
The American Action Forum, a conservative research organization, attempted to estimate the cost of deporting all of the country's undocumented immigrants, while deporting new undocumented immigrants who arrive. Doing so would take 20 years and cost between $420 billion and $619 billion, the group concluded. The group also predicted that removing a large and important segment of the American workforce from the country would reduce the size of the economy by 5.7 percent after two decades.
Trump's plan would triple the number of Immigration and Customs Enforcement officers, but it does not discuss how to pay for the additional jail beds that would be necessary if federal agents were to arrest three times as many undocumented immigrants. Nor does it discuss the administrative costs that would come with processing their cases.
"You need prosecuting attorneys, and you need enough judges and magistrates," said Thad Bingel, who served as the chief of staff of Customs and Border Protection in the Bush administration.
Though some details were missing, Bingel said that Trump's discussion of enforcement in the interior was a realistic discussion of the kinds of policies that would be needed to reduce the undocumented population. Focusing on even stricter security at the border wouldn't be enough, Bingel argued.
Trump would also require employers to verify their workers' eligibility to work and would rescind President Obama's executive actions granting a reprieve from deportation to undocumented immigrants who have children who are U.S. citizens or who were brought here illegally as children by their own parents. Those actions are held up in the federal courts for now.
3. What about new immigrants who want to come in?
In his plan, Trump also calls again for building a "wall" along the border, without providing details.
"I'd like to see more specifics," Bingel said of the proposed wall.
Trump's goal isn't simply to reduce the number of immigrants who enter illegally, though. He also aims to make it harder for immigrants who follow the rules to get visas.
The section of the plan dealing with legal immigration is at odds with Trump's statements earlier this month at the Republican primary debate. "I don't mind having a big beautiful door in that wall so that people can come into this country legally," Trump said.
His plan would raise the prevailing wage for skilled workers on H-1B visas, making them more expensive for U.S. employers to sponsor, and impose a requirement that businesses hire American workers before requesting an H-1B.
Trump would require all immigrants to show that they are able to support themselves before entering the country, and he would place a moratorium on new green cards issued to workers abroad.
4. How would Donald Trump pay for it?
In the plan, Trump accuses Mexico's leaders of "using illegal immigration to export the crime and poverty in their own country" to the United States, and he promises to force Mexico to pay to construct a wall.
In this plan, Trump calls again for tariffs on Mexican goods as well as for fees on people crossing the border in order to coerce Mexico's cooperation. Trump has said that the North American Free Trade Agreement, which went into effect in 1994, has been "a disaster." The agreement eliminated a number of tariffs and called for more barriers to be lifted over time.
"This kind of posturing with regard to Mexico isn't helpful," Krikorian said.
Since 2007, as many undocumented migrants have been returning to Mexico as have been entering the United States across the southwest border, according to Pew.