Commentary on the Rick Perry-Mitt Romney showdown at the GOP debate has focused on Perry’s “Ponzi scheme” critique of Social Security. It’s an important issue, and Perry will have to rethink the way he talks about Social Security in coming days. But not enough attention has been paid to Perry’s performance on an issue that could be his undoing in the GOP race: immigration.

The bottom line of Wednesday’s debate is that Perry, governor of a border state for more than a decade, didn’t have much to say about immigration. When asked how he would secure the U.S.-Mexican border, his answer was “boots on the ground” — Border Patrol agents — and more unmanned drone aircraft. That was about all Perry had to say, beyond criticizing President Obama. “For the president of the United States to go to El Paso, Texas, and say that the border is safer than it’s ever been, either he has some of the poorest intel of a president in the history of this country, or he was an abject liar to the American people,” Perry said. “It is not safe on that border.”