President Donald Trump may not have created the law under which families are separated at the U.S.-Mexico border, but he had to at least be seen as trying to fix it to get anything done on the immigration front, much less funding for a border wall.

The latest controversy over children being temporarily taken from parents who are arrested for illegally entering the country illustrates both why a comprehensive immigration reform bill is needed, and why it’s so difficult to achieve.

First, some facts as I best understand them. When an undocumented immigrant is arrested at the border, he or she is taken into custody by the U.S. Marshals and the children are housed at shelters by the Department of Health and Human Services. If the arrest is simply for illegal entry, that’s typically resolved fairly quickly with the parent and child being deported together.

The problem arises when the arrested parent asks for asylum, which can take months to adjudicate. Under a 1997 law called the Flores Consent Decree, unaccompanied children can only be held by the government for 20 days. After that, children are placed with a responsible party in the U.S., often a relative, while the parents await their fate. The exception is if a family seeks asylum at a U.S. consulate or port of entry, in which case families are kept together, government officials say.

While this practice pre-dated the Trump presidency, the reason you’re hearing more about this now is because the Justice Department stepped up its prosecutions for simple border-crossings as part of its “zero tolerance” policy against illegal immigration. Among other things, the policy has the potential to ensnare more would-be farm workers who just wanted their families with them.

Supporters of the policy argue that the only alternative would be to release the families into the U.S. population while they await their cases, and historically only about one-third of these adults showed up for their court dates after being freed. Further, they say American parents are separated from their children every day when they’re arrested or sent to prison.

But images of wailing children in holding shelters have provided ample photo ops for Democrats and like-minded media pundits to accuse Trump of being heartless, to the point where even some of his biggest supporters are calling timeout. Political grandstanding at the border certainly isn’t new, and both sides do it. Remember when Sen. Ted Cruz and Glenn Beck went there in 2014 with soccer balls and teddy bears for the thousands of unaccompanied children who were arriving and being held? Now Cruz is floating a bill to stop the separations.

The point is that all of these problems could be addressed in a comprehensive immigration bill. But lawmakers are less likely to come to the table if they think they can exploit a perceived humanitarian crisis caused by the other party.