On Monday, I pointed out that a majority of Republican voters were to Donald Trump's right on immigration: Fully 63 percent of them wanted a plan to deport all unauthorized immigrants. On Thursday, Trump pretty much closed the gap, proposing to deport all 11 million unauthorized immigrants currently in the US and then let some select few return.

From CNN (via Greg Sargent at the Washington Post):

Trump said Wednesday in an interview with CNN’s Dana Bash that as president he would deport all undocumented immigrants and then allow the "good ones" to reenter the country through an "expedited process" and live in the U.S. legally, though not as citizens. "Legal status," Trump suggested. "We got to move ’em out, we’re going to move ’em back in if they’re really good people."…

What Trump is actually proposing

Because this is Donald Trump we're talking about, the proposal is more of a rant than a white paper. So there are a couple different ways to interpret what he means.

Maybe Trump is just proposing a variation on something that's been proposed by Republicans before. Back in 2007, during the Bush-era fights over comprehensive immigration reform, some Republicans floated the idea of "touchback" — requiring unauthorized immigrants in the US to return to their home countries and apply for legal status from there.

Advocates said that touchback would be a disaster because immigrants wouldn't leave the country if they worried they'd be barred from returning. Indeed, right now there might be hundreds of thousands of immigrants in the US who'd be eligible for green cards through their spouses or children, but haven't applied because they think they'd have to leave (although the Obama administration made it easier for them to apply while staying in the US). But it was still treated as part of the plan for ultimate legalization and citizenship for unauthorized immigrants — because there were clear standards that would allow millions of people to get legal status and return.

Trump's proposal, on the other hand, is being treated as a plan for mass deportation. That's because it sounds like he wouldn't be letting many people back in at all. "Really good people" is a vague standard. But it's also the same language that Trump has used when talking about what immigrants aren't: in the early stages of his campaign, his immigration platform was limited to the contention that "Mexico isn't sending their best people" to immigrate to the US.

Why Republicans are so reluctant to endorse mass deportation

So if mass deportation is so popular among Republican voters, why hasn't any Republican presidential candidate — or policymaker — embraced it before now? Simple: It is a totally impractical proposal, and anyone who might actually be elected president has good reason to be wary of making it.

Let's start with cost. It's huge. The center-right (but pro–immigration reform) American Action Forum estimated earlier this year that "fully enforcing current law" — that is to say, deporting all 11 million people who are deportable — would cost somewhere between $400 billion and $600 billion. That's the net worth of 40 to 60 Donald Trumps, if you believe Trump's own estimate of his net worth — or 100 to 150 Donald Trumps, if you believe Forbes. And it's an expensive government program for the party of low taxes and small government to propose.

There's also a question of efficacy. The majority of unauthorized immigrants living in the US have been here for more than a decade. Millions of them have US-born children. Those facts make it more destructive to their communities to deport them. They also make it harder to find and deport them, period.

In Trumpworld, such considerations are secondary, because real leaders can get things done by sheer will:

Trump would not say how he would locate, round up and deport the 11 million undocumented immigrants he says must go. Instead, he deflected, saying that while it may be a task too tall for politicians, it isn’t for a business mogul like himself. "Politicians aren’t going to find them because they have no clue. We will find them, we will get them out," Trump said. "It’s feasible if you know how to manage."

But for policymakers — particularly members of Congress — they're very serious problems. If Trump endorses mass deportation, it can easily be dismissed as Trump being Trump. But if a member of Congress endorses mass deportation, he's going to be expected to come up with a plan. And how the hell is he going to do that?

Will other Republican candidates be compelled to weigh in?

In the past, Republican politicians who want all unauthorized immigrants to get out of the country, but don't want to be on the hook for figuring out how to do it, have settled on the doctrine of "self-deportation": If the government passes laws that make it sufficiently miserable to be an unauthorized immigrant in the United States — by making it impossible for him to get a job; by keeping homeowners from renting to her; or by allowing or encouraging local police to arrest him and turn him over to federal agents for deportation — the immigrant will decide to pick up and leave. Multiply that by several million self-deporting immigrants, and you get the result that supporters of mass deportation want, while saving the government tens of thousands of dollars a head.

But Republican voters aren't particularly passionate supporters of self-deportation. They appear to prefer mass deportation itself — at least at present. And while in previous cycles they might have had to settle for something less, they now have a candidate who is expressing their viewpoint relatively directly.

Of course, that candidate is Donald Trump, who is still not going to win the Republican nomination and whom other candidates have been trying valiantly not to dignify with a serious response. But that can't last. We're a week out from the first Republican debate, at which Trump is (if current polls hold) going to literally take center stage. Jeb Bush and Scott Walker can ignore Trump asking if they support mass deportation — but it's much harder for them to ignore a debate moderator asking the same question.

That puts more serious Republican candidates in a difficult spot. If they disagree with Trump, they'll pit themselves against the Republican base — and face pressure to explain what they'd do instead. If they agree with Trump, they'll be making a policy promise that will be very difficult to keep. And they'd also be destroying any chance of Latino support in the general election in 2016.