(Erin Scott/Reuters)

Many have been touting an estimate from the Migration Policy Institute that Trump’s latest proclamation — which requires many immigrants to prove they will get health insurance or can cover their own health expenses — “has the potential to block fully two-thirds of those who apply for legal permanent residence from abroad.” The devil is in the details, though, so don’t get outraged or start cheering quite yet.

First of all, that “from abroad” is significant. A lot of people applying for green cards are already here on “non-immigrant” visas and aren’t affected by the policy. Two-thirds of those applying from abroad — 375,000 — is actually just about one-third of the million-plus green cards we give out each year. (This was lost on Vox.)

Second, the estimate comes from the fact that about a third of people who recently got green cards are uninsured, while another third get taxpayer-subsidized insurance that doesn’t qualify under the policy. In other words, it assumes that no one will get insurance or switch plans to comply, which is absurd.

How easy it is to comply will depend on a lot of things we don’t know, however, including how the administration decides to implement the rather vague proclamation, how quickly insurers act to provide cheap plans designed to meet the requirements at minimal cost, and how quickly blue states move to allow immigrants to buy cheap plans those states have limited or banned.

Third, some of the people kept out by this policy could simply be replaced by others. In categories with numerical limits there are often huge backlogs, so as MPI notes, it “remains to be seen by how much the proclamation would cut overall immigration versus shifting the composition of arriving immigrants.”

To put it bluntly, we have no idea how much legal immigration will fall when the policy goes into effect next month, but it won’t be two-thirds, and it probably won’t be very close to one-third either.

Last, a word on the politics here: Something like this could have been an aggressive bargaining chip a few years ago, a way to pressure liberals in Congress to address the issue of immigration in a way Trump found acceptable. Right now, though, I imagine Democrats’ response will be to tie it up in court for as long as possible and hope to win the presidency so they can reverse it.