A myriad of logistical and legal obstacles await President Donald Trump if he follows through on his threat to place migrants arriving at the border into so-called sanctuary cities, a move apparently designed to punish Democrats for refusing to support his immigration policies.

Not only is the idea impractical and a misuse of government resources, experts said Friday, it also is illegal.

“Homeland security dollars are supposed to be used to protect national security, not go after your political opponents,” said Kerri Talbot, an attorney and federal advocacy director for the advocacy group the Immigration Hub.