If Trump wants the freedom to use the National Guard as he pleases in Chicago, the easiest legal path is to invoke the Insurrection Act — a vaguely worded 1792 law that allows the president to deploy federal troops inside the U.S. to conduct law enforcement, an exception to the restrictions of the Posse Comitatus Act.

Democratic elected officials in cities targeted by Trump have publicly rejected suggestions that their residents need the National Guard. “Crime is at its lowest point in decades, visitors are coming back, and San Francisco is on the rise,” Mayor Daniel Lurie said.

President Donald Trump's suggestion that he would send National Guard troops to Democratic-led Baltimore and Chicago riled up Democratic senators, while some Republicans in states with high-crime blue cities welcomed the idea.