This is not the first time Trump has tried to pass the buck to Congress. In general, the political squeeze is the same each time: The president’s base demands things he promised as a candidate, but the electorate as a whole opposes them. Rather than risk enraging his base, Trump has punted to Congress, on issues from health care to the Deferred Action for Childhood Arrivals program.
In some of these cases, Trump has found a pretext. He argued, for example, that he supported DACA’s aims but believed Obama’s executive order to be unconstitutional. The president said that only congressional action could make DACA legitimate and canceled it, knowing that Congress was unlikely to act.
In the case of the border separations, however, matters are much simpler. Trump claims to dislike what is happening. There’s no constitutional barrier to overturning the policy announced in May. There’s no law that has to be circumvented. All it would take would be a call from Trump, opting not to lock up every person caught crossing the border, and the separations would stop.