In September Elon Musk, a South African-born American whose firms employ many foreign engineers, posted on his website X that “the legal immigration process in America needs to be greatly streamlined and expanded.

But reshaping immigration policy — and sending hundreds of thousands of people out of the country at minimum — won’t be as simple as putting a pen to paper and ordering it to be done. Pro-immigration groups around the country are ready to stand up legal defenses.