It’s long past time to close off the border. After all, the Pew Hispanic Center estimates there are 12 million illegal immigrants in the U.S. and that 40 percent (4.4 million) have been here less than five years. Thus they’ve arrived since Sept. 11. We have no idea who these people are, of course, or where they’re living.

What we do know is that there are a virtually limitless number of potential terrorist targets in the U.S. and that it’s impossible to protect all of them. At any time, a suicide bomber can detonate on a commuter train or in the middle of a busy store.

We’ve been protected so far by good police work -- authorities have broken up dozens of terrorist plots -- and by the fact that it’s difficult for our terrorist enemies to convince people to kill themselves. Simply put, self-detonation isn’t a growth industry.

But let’s suppose that a mere one-half of 1 percent of the illegals who’ve come here in recent years have terrorist leanings. That’s still 22,000 people. Considering it took only 20 terrorists to kill 3,000 Americans, that’s a scary number.

It wouldn’t take much to bring our economy to a halt. Suppose a handful of terrorists detonated at malls in Florida, Maine, Oregon and Texas. That would probably be enough to keep people out of shopping centers for months. Or if the same attackers blew themselves up commuter trains in San Francisco, D.C., Atlanta and Boston. Commuters would stay off public transit for weeks. We need to seal the border so we can keep the number of illegals from growing. That’s the first step. The next step is to shrink the number who are already here. That process will begin when we crack down on those who employ illegals.

The fact is that most illegals are coming here to work. Take away the jobs and they’ll return home. Again, lawmakers have already passed the necessary laws. They simply need to make sure those laws, including the REAL ID Act, are fully enforced. Let’s make it easier for employers to verify the immigration status of workers, and let’s hold those employers accountable when they don’t.

Many senators seem to think they need to pass another law before we can do anything about illegal immigration. But that’s not correct. We need to enforce the laws we already have. Only when we’re doing that should we even consider passing new laws.