Congress has the power of the purse. Failure to appear: no paycheck. Multiple failure to appear: automatic departmental budget cut, ultimately to zero. The executive may choose to illegally shuffle money around but that doesn't work for long.
Ironically the legislation that created DHS (a big mistake made opportunistically at the time) also included a GOP wish that made it not subject to the same set of employee rules the rest of the civil service is.* So DHS is the easiest to do this with.
* It is still partially unionized, though the GOP tried to eliminate this when forming the department as well.
Ironically the legislation that created DHS (a big mistake made opportunistically at the time) also included a GOP wish that made it not subject to the same set of employee rules the rest of the civil service is.* So DHS is the easiest to do this with.
* It is still partially unionized, though the GOP tried to eliminate this when forming the department as well.