They cannot fight him or stop him. The president is within his full authority to ban, round up and ship home or lock up ANY non citizen of America if he believes they pose a security threat. He needs no permission from anyone and needs to supply no proof to anyone
FDR did it in WW2 and Carter did it in the 70’s when he banned Iranians from entering the country and sent thousands who were here home.
The USSC ruled on the presints authority too due this.
Thanks to Harry Reid, Obama has had a couple of years to pack the lower courts with Lefty activists.
Trump may experience a setback at the circuit court level, though I expect he’ll ultimately prevail.