The president is also on shaky legal ground to demand that local authorities, including police and jail officials, enforce federal immigration law, just as he would by insisting they enforce the federal tax code, federal environmental regulations or federal food and drug rules. In 2014, the U.S. Court of Appeals for the Third Circuit in Philadelphia ruled that detention requests made by the federal government to localities specifically, that they hold undocumented immigrants are just that: requests, not commands.
It remains unclear which federal funding Trump would or legally could try to suspend as a means of coercing jurisdictions to cooperate with federal officials, and whether such a threat would amount to much leverage. The Supreme Court has also ruled that there is no legal basis by which the president, or Congress, could withhold federal funding to localities that is unrelated to immigration for instance, for housing or hospitals. Assuming that's the case, then the president's leverage with most localities would be limited.
Nor is it clear which of the more than 300 cities and counties that withhold some form of cooperation from federal immigration officials would fall into Trump's definition of a sanctuary jurisdiction. Some localities, notably San Francisco, refuse almost all forms of cooperation. Others ignore detention requests from U.S. Immigration and Customs Enforcement for undocumented immigrants convicted of minor crimes but not those found guilty of serious or violent crimes. Many others simply instruct their police not to ask detained suspects about their immigration status.
No doubt, there are instances of localities having behaved with outrageous irresponsibility by refusing to turn over repeat offenders to ICE. That was the case, in 2015, when San Francisco refused to honor an ICE detainer for an undocumented immigrant with an extensive criminal record, who, shortly after he was released, shot and killed a young woman strolling on the waterfront.
Still, Trump stands to gain very little by declaring what amounts to a culture war on huge swaths of urban America that, with good reason, would defy his attempts to deport millions of productive and largely law-abiding immigrants, many of whom have children and other relatives who are U.S. citizens.