This should be taken with a huge grain of salt.

First off, Trump’s citing no specific authority that would allow him to unilaterally do this. Secondly, he’s not specifying which type of immigration he’s supposedly shutting down or what happens to immigrants already in the U.S. https://twitter.com/realdonaldtrump/status/1252418369170501639

“I’m not going to comment or speculate on substance unless or until there is some,” a prominent attorney who has previously challenged the Trump administration in court over immigration issues texted me tonight after I asked about the president’s tweet.