Donald Trump wants a border fence, or possibly a wall. (But one with a big door.) John Kasich also wants a fence.
Marco Rubio doesn't not want a fence. But he knows that border security isn't actually as simple as "build a fence":
"I also think we need a fence, but if El Chapo can build a tunnel under that fence, we need to deal with that."
The US has been building up its border for the last 20 years. The main lesson we've learned is that people go wherever enforcement isn't. That applies to unauthorized immigrants who aren't engaged in criminal activity, but it also applies to cartel workers smuggling people or drugs into the US. When border agents stack up on easier-to-cross sectors, people cross through punishing desert. When the US tries to build a fence, people cut holes in the fence. When the US sends surveillance drones to monitor the border from the sky, the Sinaloa cartel digs complicated tunnels to ship drugs into the US without detection.
When I talked to border experts about Jeb Bush's border security plan earlier this week, they admitted that there isn't a quick technological fix to detecting tunnels. They also said that tunnels were just one of many techniques that cartels use to get drugs into the US — because they're smart organizations that are spending a lot of time trying to figure out how to evade Border Patrol.
The US has already fixed the easy problems when it comes to the border. Apprehensions are way down, and a lot of people who come are deliberately seeking out Border Patrol agents to seek asylum (which they're legally allowed to do). There's not a ton more it can do to prevent unauthorized migration at the border — something Jeb Bush's border plan, released earlier this week, also understands.
Where there is more that could be done, as Rubio acknowledged, is interior enforcement — like E-Verify, which would require employers to check the immigration status of potential workers. Whether E-Verify would work while there are still 11 million unauthorized immigrants (many of them long-resident) in the US is an open question. But it's something that can be done.
But that's not going to help prevent cartels from smuggling in drugs. Security against smuggling is a lot harder. And while there's no one solution, it certainly requires knowing more about the border than just building a fence.