On Donald Trump’s long list of bête noires, “sanctuary cities” are near the top. And he’s promised to act on his ire.

In a major immigration speech in August, the president-elect vowed to withhold all federal funding from cities and states that don’t actively particulate in deportation campaigns. Securing their compliance is the only way Trump can hope to carry out his campaign plank of “immediately” expelling up to 3 million immigrants — at least short of massively expanding the federal immigration agency, which only has about six thousand employees.

If Trump gets his way, not only will cities have to turn over records that detail their residents’ immigration statuses (such as the municipal IDs that New York started issuing last year, or the IDs San Francisco introduced in 2009), but they could even be required to hold people without warrants or formal charges on the federal government’s behalf — or risk losing critical revenue.