Could Donald Trump deport 12 million illegal aliens?
The short answer is no, Donald Trump won't be able to deport 12 million illegal aliens. That, of course, assumes Trump is nominated and elected, and both of those aren't very likely. It is very likely that Trump knows he wouldn't be able to deport millions of illegal aliens, he's simply playing to his base.
If someone sincerely thinks Trump would be able to deport millions of illegal aliens, the best I can say about them is that they're completely ignorant of how things work and they aren't in full touch with reality. Because of the issues the Donald Trump base has, nothing I or anyone can say would educate them about how things work and about reality.
Trump's support for mass deportations is bad for the opposition to illegal immigration. He's promoting something that will never happen, while ignoring a plan that could be made to happen. If Trump really wanted to deal with the illegal immigration issue, he'd use better arguments and he'd promote the attrition plan. Under that plan, we'd ramp up immigration enforcement and reduce the benefits illegal aliens receive, causing many to leave on their own. Hundreds of thousands of illegal aliens have already left the U.S. on their own due to the economic downturn; they made the logical decision that going home was better for them than staying in the U.S. That's the idea behind attrition. It's much easier to achieve than mass deportations, without all of the many negative issues involved in rounding millions of people up.
Trump's followers are therefore supporting something that won't happen and ignoring something that could happen. By enabling Trump on this issue, they're helping those who support illegal immigration. Extensive experience with the Tea Parties has shown that they won't be able to understand that or accept that. They'll also never accept responsibility once what they supported has been shown to have been unrealistic.
Assuming that Trump is nominated and elected, a good part of the U.S. and the political establishment is going to oppose everything he does. He's not going to simply be able to give orders and expect them to be followed as he does in business. If Trump does try to start mass deportations as president, his opposition will do everything they can to stop him. Apparently Trump supporters think that his opponents wouldn't be able to do anything, or they'd just let Trump do what he wants. The entire history of illegal immigration shows that's not at all realistic. Trump points to Eisenhower's efforts in 1954 , and that's a bit of a tell: Trump knows the U.S. today is far different from what it was in the 50s. He knows that no president could get away with the things that Eisenhower and past presidents were able to get away with today.
The first thing that would happen is that the media would begin running article after article, segment after segment portraying Trump's plan in the worst light possible. That would turn some millions of people against it. Further, even if the media outright lied about Trump's plans, his supporters wouldn't be able to do anything about it. The types of people who support Trump simply lack the will and the ability to do anything about media bias, as everything that's happened over the past decades shows.
At the same time, the GOP establishment would begin pressuring or threatening their members to oppose Trump's plans. They might, for instance, tie supporting a candidate in future elections to opposing Trump. Whether rightly or wrongly, the GOP leadership would think that Trump's plans would greatly harm their outreach to Hispanics. Few GOP in Congress who supported Trump's plan (or who at least initially pretended to support it) would be able to hold out against the wishes of their party, very strongly expressed. There are maybe 50 Congressmen associated with the Teaparties right now, and that number isn't likely to change much. Would all of them support Trump's plans, even though their leadership was, for instance, threatening to withhold support from their reelection efforts? The rest of Congress would be Democrats or "RINOs", and they'd oppose Trump.
Congress could stop Trump's plans, such as by withdrawing funding or even more severe actions. Only a small part of Congress would be on Trump's side. The rest would find it much easier - and much better for their careers - to oppose Trump.
Those on the fence would be approached by lobbyists from the US Chamber of Commerce and other business groups, as well as their other business supporters, any billionaire benefactors they might have, and so on. Those very wealthy forces would make it clear that supporting Trump's ideas meant no more money. It wouldn't take long before Trump had very few supporters in Congress, and they'd move to stop his plans.
At the same time as all that was happening, the American Civil Liberties Union, National Council of La Raza, LULAC, NALEO, and perhaps even the Mexican government would file legal action after legal action that would at the least temporarily halt Trump's plans (just as they did with the Arizona immigration law).
Add in the banking community (see immigration banks) which Trump relies on to do his business deals. Major banks and the Federal Reserve profit from illegal immigration, and they aren't going to let Trump cost them money.
Almost all religious leaders would oppose Trump's plan, not just far-lefties like Jim Wallis. They'd move their parishioners to oppose Trump's plans, and many would respond such as by contacting their representatives.
Even the United Nations would get involved, causing foreign countries and foreign media to portray the Trump administration in the worst light possible.
At the end of the day, Trump will never been able to follow through on his plans. And, that might be his goal. He could then tell his followers "I tried" and begin giving work permits to illegal aliens. Even if, like Obama, Trump didn't try to mass legalize illegal aliens, millions of illegal aliens would remain in the U.S.
If Trump supporters had simply demanded that he support attrition instead, we could have reduced the number of illegal aliens in the U.S. Demanding a realistic plan would have been better for the U.S. than believing in a fairy tale, but Trump's supporters lack the ability to understand that.
 From the 11/10/15 debate:
TRUMP: Let me just tell you that Dwight Eisenhower, good president, great president, people liked him. "I like Ike," right? The expression. "I like Ike." Moved a 1.5 million illegal immigrants out of this country, moved them just beyond the border. They came back.
Moved them again beyond the border, they came back. Didn't like it. Moved them way south. They never came back.